ESS TECH, INC., 10-Q filed on 5/15/2025
Quarterly Report
v3.25.1
Cover - shares
 3 Months Ended
Mar. 31, 2025
May 12, 2025
Entity Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Mar. 31, 2025  
Document Transition Report false  
Entity File Number 001-39525  
Entity Registrant Name ESS Tech, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 98-1550150  
Entity Address, Address Line One 26440 SW Parkway Ave.  
Entity Address, Address Line Two Bldg. 83  
Entity Address, City or Town Wilsonville  
Entity Address, State or Province OR  
Entity Address, Postal Zip Code 97070  
City Area Code 855  
Local Phone Number 423-9920  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   12,103,750
Entity Central Index Key 0001819438  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q1  
Current Fiscal Year End Date --12-31  
Amendment Flag false  
Common Stock    
Entity Information [Line Items]    
Title of 12(b) Security Common stock, $0.0001 par value per share  
Trading Symbol GWH  
Security Exchange Name NYSE  
Warrants    
Entity Information [Line Items]    
Title of 12(b) Security Warrants, each fifteen warrants exercisable for one share of common stock at an exercise price of $172.50  
Trading Symbol GWH.W  
Security Exchange Name NYSE  
v3.25.1
Condensed Balance Sheets (unaudited) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Current assets:    
Cash and cash equivalents $ 8,422 $ 13,341
Restricted cash, current 906 906
Accounts receivable, net 253 215
Short-Term Investments 4,379 18,263
Inventory 6,884 5,641
Prepaid expenses and other current assets 2,848 4,998
Total current assets 23,692 43,364
Property and equipment, net 22,510 20,582
Intangible assets, net 4,589 4,656
Operating lease right-of-use assets 1,142 1,503
Restricted cash, non-current 868 948
Other non-current assets 780 760
Total assets 53,581 71,813
Current liabilities:    
Accounts payable 10,215 8,070
Accrued and other current liabilities 7,351 9,315
Accrued product warranties 2,028 3,288
Operating lease liabilities, current 1,293 1,692
Deferred revenue, current 1,359 5,237
Total current liabilities 22,246 27,602
Deferred revenue, non-current 18,200  
Common stock warrant liabilities 917 802
Other non-current liabilities 104 125
Total liabilities 41,502 42,929
Commitments and contingencies (Note 8)  
Stockholders’ equity:    
Preferred stock ($0.0001 par value; 200,000,000 shares authorized, none issued and outstanding as of March 31, 2025 and December 31, 2024) 0 0
Common stock ($0.0001 par value; 1,000,000,000 shares authorized, 12,103,750 and 11,986,516 shares issued and outstanding as of March 31, 2025 and December 31, 2024, respectively) 1 1
Additional paid-in capital 812,483 811,262
Accumulated deficit (800,405) (782,379)
Total stockholders’ equity 12,079 28,884
Total liabilities and stockholders’ equity 53,581 71,813
Nonrelated party    
Current liabilities:    
Deferred revenue, non-current 3,835 0
Related party    
Current liabilities:    
Deferred revenue, non-current $ 14,400 $ 14,400
v3.25.1
Condensed Balance Sheets (unaudited) (Parenthetical) - $ / shares
Mar. 31, 2025
Dec. 31, 2024
Statement of Financial Position [Abstract]    
Preferred stock, par value (in USD per share) $ 0.0001 $ 0.0001
Preferred stock, shares authorized (in shares) 200,000,000 200,000,000
Preferred stock, shares outstanding (in shares) 0 0
Preferred stock, shares issued (in shares) 0 0
Common stock, par value (in USD per share) $ 0.0001 $ 0.0001
Common stock, shares authorized (in shares) 1,000,000,000 1,000,000,000
Common stock, shares issued (in shares) 12,103,750 11,986,516
Common stock, shares outstanding (in shares) 12,103,750 11,986,516
v3.25.1
Condensed Statements of Operations and Comprehensive Loss (unaudited) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Revenue:    
Total revenue $ 599 $ 2,738
Cost of revenue 8,746 11,126
Gross profit (loss) (8,147) (8,388)
Operating expenses    
Research and development 2,478 3,546
Sales and marketing 1,950 2,034
General and administrative 5,571 5,526
Total operating expenses 9,999 11,106
Loss from operations (18,146) (19,494)
Other income, net    
Interest income, net 216 1,239
Net decrease to warrant liabilities (115) 0
Other income (expense), net 19 (55)
Total other income, net 120 1,184
Net loss to common stockholders (18,026) (18,310)
Comprehensive loss to common stockholders $ (18,026) $ (18,310)
Net loss per share - basic (in USD per share) $ (1.50) $ (1.57)
Net loss per share - diluted (in USD per share) $ (1.50) $ (1.57)
Weighted average shares used in per share calculation, basic (in shares) 12,033,442 11,634,302
Weighted average shares used in per share calculation, diluted (in shares) 12,033,442 11,634,302
Nonrelated party    
Revenue:    
Total revenue $ 571 $ 2,214
Related party    
Revenue:    
Total revenue $ 28 $ 524
v3.25.1
Condensed Statements of Stockholders' Equity (unaudited) - USD ($)
$ in Thousands
Total
Common Stock
Additional Paid-In Capital
Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2023   11,614,127    
Balance at beginning of period at Dec. 31, 2023 $ 103,357 $ 1 $ 799,513 $ (696,157)
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Issuance of common stock upon exercise of options (in shares)   45,745    
Issuance of common stock under stock-based compensation plans, net of stock withheld for taxes (81)   (81)  
Stock-based compensation expense 2,854   2,854  
Net loss (18,310)     (18,310)
Balance at end of period (in shares) at Mar. 31, 2024   11,659,872    
Balance at end of period at Mar. 31, 2024 $ 87,820 $ 1 802,286 (714,467)
Balance at beginning of period (in shares) at Dec. 31, 2024 11,986,516 11,986,516    
Balance at beginning of period at Dec. 31, 2024 $ 28,884 $ 1 811,262 (782,379)
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Issuance of common stock upon exercise of options (in shares)   117,234    
Issuance of common stock under stock-based compensation plans, net of stock withheld for taxes (13)   (13)  
Stock-based compensation expense 1,234   1,234  
Net loss $ (18,026)     (18,026)
Balance at end of period (in shares) at Mar. 31, 2025 12,103,750 12,103,750    
Balance at end of period at Mar. 31, 2025 $ 12,079 $ 1 $ 812,483 $ (800,405)
v3.25.1
Condensed Statements of Cash Flows (unaudited) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Cash flows from operating activities:    
Net loss $ (18,026) $ (18,310)
Adjustments to reconcile net loss to net cash used in operating activities:    
Depreciation and amortization 1,540 1,219
Non-cash interest income (137) (995)
Non-cash lease expense 361 326
Stock-based compensation expense 1,234 2,854
Inventory write-down and losses on noncancellable purchase commitments (774) (1,979)
Change in fair value of common stock warrant liabilities 115 0
Other non-cash expenses, net 58 29
Changes in operating assets and liabilities:    
Accounts receivable, net (19) 1,309
Inventory (390) 1,145
Prepaid expenses and other assets 2,130 (266)
Accounts payable 497 153
Accrued and other liabilities (3,106) (4,024)
Accrued product warranties (1,260) 1,193
Deferred revenue (62) (1,184)
Operating lease liabilities (399) (380)
Net cash used in operating activities (18,238) (18,910)
Cash flows from investing activities:    
Purchases of property and equipment (762) (953)
Maturities and purchases of short-term investments, net 14,014 35,645
Net cash provided by investing activities 13,252 34,692
Cash flows from financing activities:    
Proceeds from stock options exercised 4 0
Repurchase of shares from employees for income tax withholding purposes (17) (81)
Net cash used in financing activities (13) (81)
Net change in cash, cash equivalents and restricted cash (4,999) 15,701
Cash, cash equivalents and restricted cash, beginning of period 15,195 22,483
Cash, cash equivalents and restricted cash, end of period 10,196 38,184
Supplemental disclosures of cash flow information:    
Cash paid for operating leases included in cash used in operating activities 438 449
Non-cash investing and financing transactions:    
Purchase of property and equipment included in accounts payable and accrued and other current liabilities 4,277 517
Transfers between inventory and property and equipment, net 0 1,051
Cash and cash equivalents 8,422 36,332
Restricted cash, current 906 906
Restricted cash, non-current 868 946
Total cash, cash equivalents and restricted cash shown in the condensed statements of cash flows $ 10,196 $ 38,184
v3.25.1
DESCRITPION OF BUSINESS AND BASIS OF PRESENTATION
 3 Months Ended
Mar. 31, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]  
DESCRITPION OF BUSINESS AND BASIS OF PRESENTATION DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION
Description of Business—ESS Tech, Inc. (“ESS” or the “Company”) is a long-duration energy storage company specializing in iron flow battery technology. ESS develops long-duration iron flow batteries for commercial and utility-scale energy storage applications requiring up to twelve hours of flexible energy capacity predominantly using earth-abundant materials.
The Company was founded in 2011 (“Legacy ESS”) and became a publicly traded company through a business combination with a special purpose acquisition company named ACON S2 Acquisition Corp. (“STWO”) which changed its name to ESS Tech, Inc. upon closing (the “Business Combination”). As a result of the Business Combination, Legacy ESS survived and became a wholly owned subsidiary of ESS Tech, Inc. On March 31, 2024, Legacy ESS merged with ESS Tech, Inc. leaving ESS Tech, Inc. as the sole remaining legal entity. As of April 1, 2024, the Company did not have any subsidiaries.
Basis of Presentation—The accompanying condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”).
Condensed Financial Statements—The accompanying unaudited condensed financial statements have been prepared in accordance with U.S. GAAP for interim financial information and in accordance with the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) for interim financial reporting. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. The condensed financial statements reflect all normal and recurring adjustments that are, in the opinion of the Company’s management, necessary in order to make the condensed financial statements not misleading. Operating results for the three months ended March 31, 2025 are not necessarily indicative of the results that may be expected for the year ending December 31, 2025. These condensed financial statements should be read in conjunction with the financial statements and related notes included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2024, as filed with the SEC on March 31, 2025.
Reclassifications—Certain immaterial prior year amounts have been reclassified to conform with the current year presentation. These reclassifications had no effect on the reported results of operations.
Liquidity and Capital Resources—The Company has incurred operating losses and cash outflows from operations since inception and the Company anticipates that losses will continue in the near term. During the three months ended March 31, 2025, the Company incurred net losses of $18.0 million and used $18.2 million of cash in operating activities. As of March 31, 2025, the Company had unrestricted cash and cash equivalents of $8.4 million and short-term investments of $4.4 million, or total liquid assets of $12.8 million.
The continuation of the Company as a going concern is dependent upon generating profit from its operations and its ability to obtain additional debt or equity financing. Management is evaluating various strategies to obtain additional funding, which may include additional offerings of equity, issuance of debt, or other capital sources. There is no assurance that the Company will be able to generate sufficient profits, obtain such financings, or obtain them on favorable terms. Any such financing activities are subject to market conditions and accordingly involves factors that are outside the Company’s control. These uncertainties cause substantial doubt to exist as to the Company’s ability to continue as a going concern for 12 months from the issuance of these financial statements. The accompanying condensed financial statements have been prepared on a going concern basis, which contemplates the realization of assets and satisfaction of liabilities in the normal course of business. The condensed financial statements do not reflect any adjustments that might result from the outcome of this uncertainty.
v3.25.1
SIGNIFICANT ACCOUNTING POLICIES
 3 Months Ended
Mar. 31, 2025
Accounting Policies [Abstract]  
SIGNIFICANT ACCOUNTING POLICIES SIGNIFICANT ACCOUNTING POLICIES
The Company’s significant accounting policies have not changed from those disclosed in the annual audited financial statements and accompanying notes in the Annual Report on Form 10-K for the fiscal year ended December 31, 2024.
Reverse Stock Split
On August 23, 2024, the Company filed a certificate of amendment to the Company’s Certificate of Incorporation with the Secretary of State of the State of Delaware to effect a reverse stock split of all shares of the Company’s common stock that were issued and outstanding at a ratio of 1-for-15 ( the “Reverse Stock Split”) and reduce the total number of authorized shares of common stock from 2,000,000,000 to 1,000,000,000. The amendment became effective as of 4:01 p.m., Eastern Time, on August 23, 2024. The par value of the Company’s common stock remained unchanged at
$0.0001 per share. Proportionate adjustments were made to the number of shares issuable upon the exercise or vesting of all warrants, RSUs (as defined herein) and options outstanding at the effective time of the Reverse Stock Split, as well as to their corresponding exercise prices. All share-based balances, including warrants, RSUs and options, herein are reported on a retroactively adjusted basis.
Recently Issued Accounting Pronouncements—Not Yet Adopted
In November 2024, the FASB issued ASU No. 2024-03, Income Statement-Reporting Comprehensive Income (Topic 220): Expense Disaggregation Disclosures. The ASU requires entities to disaggregate operating expenses into specific categories within the notes to the financial statements to provide enhanced transparency. The ASU will be effective for fiscal years beginning after December 15, 2026, with early adoption permitted, and may be applied retrospectively or prospectively. The Company is currently evaluating the effect of this new standard on the Company's disclosures.
In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This new standard will be effective for the annual periods beginning the year ended December 31, 2025. The new standard permits early adoption and can be applied prospectively or retrospectively. We do not expect the adoption of this guidance to have a material impact on our financial statements.
v3.25.1
INVENTORY
 3 Months Ended
Mar. 31, 2025
Inventory Disclosure [Abstract]  
INVENTORY INVENTORY
Inventory consists of the following (in thousands):
March 31, 2025December 31, 2024
Raw materials$11,052 $12,084 
Work in process5,943 4,521 
Finished goods5,639 5,639 
Inventory, gross$22,634 $22,244 
Net realizable value adjustment(15,750)(16,603)
Inventory$6,884 $5,641 
The balance of the Company’s inventory was written down by $15.8 million and $16.6 million from its cost to its net realizable value as of March 31, 2025 and December 31, 2024, respectively. Additionally, the Company has lower of cost or net realizable value (“LCNRV”) losses related to noncancellable purchase commitments which were $0.3 million and $0.2 million as of March 31, 2025 and December 31, 2024, respectively. These LCNRV losses related to noncancellable purchase commitments are reflected in the materials and related purchases component of accrued and other liabilities on the condensed balance sheets. For further details, refer to Note 8, Commitments and Contingencies.
v3.25.1
PROPERTY AND EQUIPMENT, NET
 3 Months Ended
Mar. 31, 2025
Property, Plant and Equipment [Abstract]  
PROPERTY AND EQUIPMENT, NET PROPERTY AND EQUIPMENT, NET
Property and equipment, net consists of the following (in thousands):
March 31, 2025December 31, 2024
Machinery and equipment$24,295 $23,306 
Leasehold improvements6,457 6,234 
Furniture and fixtures231 231 
Software614 614 
Construction in process5,911 3,722 
Total property and equipment37,508 34,107 
Less accumulated depreciation(14,998)(13,525)
Total property and equipment, net$22,510 $20,582 
Depreciation expense related to property and equipment, net was $1.5 million and $1.2 million for the three months ended March 31, 2025 and 2024, respectively.
v3.25.1
INTANGIBLE ASSETS, NET
 3 Months Ended
Mar. 31, 2025
Goodwill and Intangible Assets Disclosure [Abstract]  
INTANGIBLE ASSETS, NET INTANGIBLE ASSETS, NET
In September 2023, the Company acquired patent rights valued at $5.0 million under a Patent License Agreement with UOP LLC (“UOP”), an affiliate of Honeywell International Inc. (“Honeywell”), a related party. These patent rights
were recorded at fair value based on the value of the IP Warrants issued, as defined in Note 9, Common Stock Warrants, and are amortized over an average useful life of 19 years based on the remaining useful lives of the patents acquired. Amortization expense for both of the three months ended March 31, 2025 and 2024 was $67 thousand.
Intangible assets, net consisted of the following (in thousands):
March 31, 2025December 31, 2024
CostAccumulated AmortizationNet Carrying AmountCostAccumulated AmortizationNet Carrying Amount
Patents$4,990 $(401)$4,589 $4,990 $(334)$4,656 
v3.25.1
ACCRUED AND OTHER CURRENT LIABILITIES
 3 Months Ended
Mar. 31, 2025
Payables and Accruals [Abstract]  
ACCRUED AND OTHER CURRENT LIABILITIES ACCRUED AND OTHER CURRENT LIABILITIES
Accrued and other current liabilities consist of the following (in thousands):
March 31, 2025December 31, 2024
Payroll and related benefits$2,266 $4,351 
Materials and related purchases2,550 2,499 
Professional and consulting fees1,566 831 
Noncancellable purchase commitments312 233 
Environmental, Health & Safety compliance liability299 702 
Other358 699 
Total accrued and other current liabilities$7,351 $9,315 
v3.25.1
ACCRUED PRODUCT WARRANTIES
 3 Months Ended
Mar. 31, 2025
Guarantees and Product Warranties [Abstract]  
ACCRUED PRODUCT WARRANTIES ACCRUED PRODUCT WARRANTIES
The following table summarizes product warranty activity (in thousands):
Three Months Ended March 31,
20252024
Accrued product warranties - beginning of period$3,288 $2,129 
Accruals for warranties issued 94 2,583
Repairs and replacements(1,306)(1,083)
Adjustments to existing accruals(48)(307)
Accrued product warranties - end of period$2,028 $3,322 
v3.25.1
COMMITMENTS AND CONTINGENCIES
 3 Months Ended
Mar. 31, 2025
Commitments and Contingencies Disclosure [Abstract]  
COMMITMENTS AND CONTINGENCIES COMMITMENTS AND CONTINGENCIES
Legal Proceedings
The Company, from time to time, is a party to various claims, legal actions, and complaints arising in the ordinary course of business. The Company is not aware of any material legal proceedings or other claims, legal actions, or complaints through the date of issuance of these condensed financial statements.
Letters of Credit
The Company had a standby letter of credit with First Republic Bank for $75 thousand as security for an operating lease of office and manufacturing space in Wilsonville, Oregon secured by a restricted certificate of deposit totaling $75 thousand. As of December 31, 2024 the certificate of deposit was recorded as restricted cash, non-current. The letter of credit expired during February 2025 and as of March 31, 2025 was no longer recorded as restricted cash. There were no draws against the letter of credit during the three months ended March 31, 2025 and 2024.
The Company has a standby letter of credit with Bank of America for $0.6 million as security for the performance and payment of the Company’s obligations under a customer agreement. The letter of credit is in effect until the date on which the warranty period under the agreement expires, which is anticipated to be more than a year from the balance sheet date. As of March 31, 2025, $0.6 million was pledged as collateral for the letter of credit and recorded as restricted cash, non-current. There were no draws against the letter of credit during the three months ended March 31, 2025 and 2024.
The Company has a standby letter of credit with Bank of America for $0.2 million in support of the Company’s customs and duties due on imported materials. The letter of credit is in effect until May 19, 2025. As of March 31,
2025, $0.2 million was pledged as collateral for the letter of credit and recorded as restricted cash, current. There were no draws against the letter of credit during the three months ended March 31, 2025 and 2024.
Credit Agreement
On November 1, 2024, the Company entered into a Credit Agreement with Export-Import Bank of the United States, as lender, and related agreements related to the financing of two production lines. The Credit Agreement provides for a secured loan facility in an aggregate principal amount of up to $22.7 million, of which $20.0 million is available to be borrowed for equipment financing and the balance will be used to finance an exposure fee and transaction expenses. The loan facility has a maturity date of June 30, 2031. Half of the proceeds of the loan facility may be used on a retroactive basis for the financing of the Company’s existing automated battery assembly line and the remainder may be used for the financing or refinancing of an additional line upon the closing of an equity raise milestone. As of March 31, 2025, the Company had no outstanding borrowings under the Credit Agreement.
Purchase Commitments
The Company purchases materials from numerous suppliers and has entered into agreements with various contract manufacturers, which include cancellable and noncancellable purchase commitments. As of March 31, 2025 and December 31, 2024, total unfulfilled noncancellable purchase commitments were $0.3 million and $0.2 million, respectively. In addition, total unfulfilled cancellable purchase commitments amounted to $4.8 million and $6.0 million as of March 31, 2025, and December 31, 2024, respectively.
Joint Development Agreement
In September 2023, the Company entered into a Joint Development Agreement (“JDA”) with UOP, an affiliate of Honeywell, a related party, under which the parties agreed to work collaboratively to engage in certain research and development activities generally related to flow battery technology. Pursuant to the JDA, the Company agreed to reimburse UOP a minimum of $8.0 million for research and development expenses incurred through December 31, 2028. No expenses were incurred under the JDA during the three months ended March 31, 2025.
v3.25.1
COMMON STOCK WARRANTS
 3 Months Ended
Mar. 31, 2025
Equity [Abstract]  
COMMON STOCK WARRANTS COMMON STOCK WARRANTS
The following table consists of the number of shares of common stock issuable upon exercise of the respective warrants as of the dates indicated:
March 31, 2025December 31, 2024
Public Warrants764,081 764,081 
SMUD Warrant833 833 
Honeywell Warrants:
Investment Warrant708,775 708,775 
IP Warrant417,997 417,997 
Performance Warrants51,717 51,717 
Total common stock warrants1,943,4031,943,403
Public Warrants
As part of STWO’s initial public offering, 8,333,287 warrants to purchase common stock (the “Public Warrants”) were sold. Simultaneously with STWO’s initial public offering, STWO issued in a private placement 4,666,667 warrants to purchase common stock (the “Private Warrants”) to STWO’s sponsor. In connection with the Business Combination, STWO’s sponsor agreed to forfeit 583,333 Private Warrants. Of the remaining 4,083,334 Private Warrants, 3,500,000 were immediately vested and 583,334 warrants (the “Earnout Warrants”) were vested upon meeting certain earnout milestone events on November 9, 2021. The Private Warrants, including the Earnout Warrants, automatically converted on a 1:1 basis into Public Warrants upon the transfer of such warrants by the initial holder to a third party during the fourth quarter of 2023.
The Public Warrants are listed on the NYSE under the ticker symbol “GWH.W.” Following the Reverse Stock Split, fifteen Public Warrants entitle the holder thereof to purchase one share of common stock at a price of $172.50 per share, subject to adjustments. The Public Warrants may be exercised only for a whole number of shares of common stock. No fractional shares will be issued upon exercise of the warrants. The Public Warrants expire on October 8, 2026, five years after completion of the Business Combination, or earlier upon redemption or liquidation.
The Company may call the Public Warrants for redemption starting any time, in whole and not in part, at a price of $0.01 per warrant, so long as the Company provides no less than 30 days prior written notice of redemption to each warrant holder, and if, and only if, the reported last sale price of common stock equals or exceeds $270.00 per share for any 20 trading days within a 30-trading day period ending on the third trading day prior to the date the Company sends the notice of redemption to the warrant holders provided there is an effective registration statement covering the shares of common stock issuable upon exercise of the warrants.
The Company may call the Public Warrants for redemption starting any time, in whole and not in part, at a price of $0.10 per warrant, so long as the Company provides no less than 30 days prior written notice of redemption to each warrant holder; provided that holders will be able to exercise their warrants on a cashless basis prior to redemption and receive a number of shares determined based on the redemption date fair market value of the shares, and if, and only if, the reported last sale price of common stock equals or exceeds $150.00 per share for any 20 trading days within a 30-trading day period ending on the third trading day prior to the date the Company sends the notice of redemption to the warrant holders provided there is an effective registration statement covering the shares of common stock issuable upon exercise of the warrants.
The Company’s common stock warrants were initially recorded at fair value upon completion of the Business Combination and are adjusted to fair value at each reporting date based on the market price of the Public Warrants, with the change in fair value recorded as a component of other income in the condensed statements of operations and comprehensive loss. For the three months ending March 31, 2025, the Company recorded a net decrease to the common stock warrant liabilities of $0.1 million. For the three months ended March 31, 2024, the Company’s common stock warrant liabilities remained constant.
SMUD Warrant
On September 16, 2022, the Company entered into a warrant agreement with the Sacramento Municipal Utility District (“SMUD”), whereby the Company agreed to issue a warrant for up to 33,333 shares of the Company’s common stock at an exercise price of $64.44 per share. The vesting of the shares underlying the warrant will be subject to the achievement of certain commercial milestones through December 31, 2030 pursuant to a related commercial agreement. As of March 31, 2025 and December 31, 2024, 833 shares underlying the warrant were vested.
Honeywell Warrants
On September 21, 2023, the Company entered into a Common Stock and Warrant Purchase Agreement (the “Purchase Agreement”) with Honeywell ACS Ventures LLC (“Honeywell Ventures”), an affiliate of Honeywell, a related party. Pursuant to the Purchase Agreement, Honeywell invested $27.5 million in the Company and the Company issued 1,099,450 shares of common stock and a warrant to issue up to 708,775 shares of common stock (the “Investment Warrant”) to Honeywell Ventures. Pursuant to the Purchase Agreement and also as further consideration for the licensing by UOP, an affiliate of Honeywell, of certain intellectual property to the Company, the Company issued a warrant to issue up to 417,997 shares of common stock (the “IP Warrant”) to UOP, which UOP subsequently transferred to Honeywell Ventures. The Investment Warrant has an exercise price of $28.35, and the IP Warrant has an exercise price of $43.50. Each warrant will expire on September 21, 2028.
On September 21, 2023, the Company and UOP also entered into a Master Supply Agreement (the “Supply Agreement”), pursuant to which UOP may purchase equipment supplied by the Company. Pursuant to the Supply Agreement, the Company agreed to issue additional warrants to purchase common stock to UOP, consisting of (i) an initial performance warrant to issue up to 51,717 shares of common stock, issued on September 21, 2023 in exchange for a prepayment of equipment by UOP in the amount of $15 million, and (ii) additional performance warrants (not to exceed an aggregate value of $15 million based on target purchase amounts of up to $300 million by 2030) to be issued on an annual basis for the five-year period beginning in 2026, based on UOP’s purchase of additional equipment after execution of the Supply Agreement (the “Performance Warrants”). The initial Performance Warrant has an exercise price of $21.75 and the additional Performance Warrants will have an exercise price equal to the volume-weighted average price of the Company’s common stock for the last fifteen (15) trading days of the relevant calendar year for which such Performance Warrant is being issued. The initial Performance Warrant will expire on September 21, 2028 and each additional Performance Warrant will have a five-year term from its respective date of issuance.
The table below summarizes the common stock warrant activities in the number of shares of common stock issuable upon exercise of the respective warrants during the three months ended March 31, 2025:
December 31, 2024IssuedExercisedMarch 31, 2025
Public Warrants764,081 — — 764,081
SMUD Warrant833 — — 833
Investment Warrant708,775 — — 708,775
IP Warrant417,997 — — 417,997
Performance Warrants51,717 — — 51,717
Total common stock warrants1,943,403 — — 1,943,403
The table below summarizes the common stock warrant activities in the number of shares of common stock issuable upon exercise of the respective warrants during the three months ended March 31, 2024:
December 31, 2023IssuedExercisedMarch 31, 2024
Public Warrants764,081 — — 764,081
SMUD Warrant833 — — 833 
Investment Warrant708,775 — — 708,775
IP Warrant417,997 — — 417,997
Performance Warrants51,717 — — 51,717
Total common stock warrants1,943,403 — — 1,943,403
v3.25.1
STOCK-BASED COMPENSATION
 3 Months Ended
Mar. 31, 2025
Share-Based Payment Arrangement [Abstract]  
STOCK-BASED COMPENSATION STOCK-BASED COMPENSATION
Stock-based compensation expense is allocated on a departmental basis based on the classification of the award holder. The following table presents the amount of stock-based compensation related to stock-based awards issued to employees on the Company’s condensed statements of operations and comprehensive loss (in thousands):
Three Months Ended March 31,
20252024
Cost of revenue$620 $924 
Research and development202 401 
Sales and marketing186 95 
General and administrative226 1,434 
Total stock-based compensation$1,234 $2,854 
2021 Equity Incentive Plan
In October 2021, the Board of Directors of the Company adopted the ESS Tech, Inc. 2021 Equity Incentive Plan (the “2021 Plan”). The 2021 Plan became effective upon consummation of the Business Combination. Stock awards under the plan may be issued as Incentive Stock Options (“ISO”), Non-statutory Stock Options (“NSO”), Stock Appreciation Rights, and Restricted Stock Awards (“RSU”). Only employees are eligible to receive ISO awards. Employees, directors, and consultants who provide continuous service to the Company are eligible to receive stock awards other than ISOs. The number of shares available for issuance under the 2021 Plan will be increased on the first day of each fiscal year beginning with the 2022 fiscal year and ending with the 2031 fiscal year, in an amount equal to the lesser of (i) 1,017,333 shares, (ii) five percent (5%) of the outstanding shares on the last day of the immediately preceding fiscal year, or (iii) such number of shares determined by the Company no later than the last day of the immediately preceding fiscal year. As of January 1, 2025, the number of shares available for issuance under the 2021 Plan was increased by 599,325 shares in accordance with the plan and as approved by the Board. Under the 2021 Plan, the Company is authorized to issue 2,353,325 shares of common stock as of March 31, 2025.
Option prices for incentive stock options are set at the fair market value of the Company’s common stock at the date of grant. The fair market value of RSUs is set at the closing sales price of the Company’s common stock at the date of grant. Employee new hire grants generally cliff vest 1/4th at the end of the first year and then vest 1/16th each quarter
over the remaining three years. Grants expire 10 years from the date of grant. All other grants vest quarterly over four years.
As of March 31, 2025, there were 1,223,746 shares available for future grant under the 2021 Plan.
Stock Options and Restricted Stock Units
Stock option and RSU activity, prices, and values during the three months ended March 31, 2025 are as follows (in thousands, except for share, per share, and contractual term data):
Options OutstandingRSUs
Number of
shares		Weighted
average
exercise price		Weighted
average
remaining
 contractual
term
(years)		Aggregate
intrinsic
values
($'000s)		Number of plan shares outstandingWeighted average
grant date fair value
per Share
Balances as of December 31, 2024
151,288 $22.00 5.09$84 1,123,890 $27.72 
Options and RSUs granted— — 41,250 4.65 
Options exercised and RSUs released(735)5.00 (120,032)18.67 
Options and RSUs forfeited(8,589)23.97 (326,755)50.28 
Balances as of March 31, 2025
141,964 $21.97 2.89$4 718,353 $17.65 
Options vested and exercisable - December 31, 2024
130,876 $20.64 4.59$84 
Options vested and exercisable - March 31, 2025
132,644 $21.43 2.52$4 
No options were granted during the three months ended March 31, 2025 and 2024.
As of March 31, 2025, there was approximately $11.1 million of unamortized stock-based compensation expense related to unvested stock options and RSUs, which is expected to be recognized over a weighted-average period of 2.12 years.
Employee Stock Purchase Plan
In May 2022, the Company commenced its first offering period under the ESS Tech, Inc. Employee Stock Purchase Plan (“ESPP”), which assists employees in acquiring a stock ownership interest in the Company. The ESPP permits eligible employees to purchase common stock at a discount through payroll deductions during specified offering periods. No employee may purchase more than $25,000 worth of stock in any calendar year. The price of shares purchased under the ESPP is equal to 85% of the fair market value of the common stock on the first or last day of the offering period, whichever is lower. As of March 31, 2025, there were 97,834 shares available for future grant under the 2021 ESPP Plan. Total ESPP expense for the three months ended March 31, 2025 and 2024 was $51 thousand and $84 thousand, respectively.
v3.25.1
FAIR VALUE MEASUREMENTS
 3 Months Ended
Mar. 31, 2025
Fair Value Disclosures [Abstract]  
FAIR VALUE MEASUREMENTS FAIR VALUE MEASUREMENTS
The following tables present the Company’s fair value hierarchy for its financial assets measured at fair value on a recurring basis (in thousands):
March 31, 2025
Cash Equivalents and Restricted CashShort-Term InvestmentsTotal Assets at Fair Value
Level 1:
Money market funds$5,386 $— $5,386 
U.S. Treasury securities— 3,180 3,180 
Total Level 15,386 3,180 8,566 
Level 2:
Certificate of deposit80 — 80 
Commercial paper— 1,199 1,199 
Total Level 280 1,199 1,279 
Total assets measured at fair value$5,466 $4,379 $9,845 
December 31, 2024
Cash Equivalents and Restricted CashShort-Term InvestmentsTotal Assets at Fair Value
Level 1:
Money market funds$7,232 $— $7,232 
U.S. Treasury securities— 7,142 7,142 
Total Level 17,232 7,142 14,374 
Level 2:
Certificate of deposit80 — 80 
Commercial paper4,811 7,825 12,636 
Corporate debt securities— 3,296 3,296 
Total Level 24,891 11,121 16,012 
Total assets measured at fair value$12,123 $18,263 $30,386 
The following tables present the Company’s fair value hierarchy for its financial liabilities measured at fair value on a recurring basis (in thousands):
March 31, 2025
Level 1Level 2Level 3Total
Liabilities:
Public common stock warrants917 — — 917 
Total liabilities measured at fair value$917 $— $— $917 
December 31, 2024
Level 1Level 2Level 3Total
Liabilities:
Public common stock warrants802 — — 802 
Total liabilities measured at fair value$802 $— $— $802 
There were no transfers among Level 1, Level 2, or Level 3 categories during the periods presented. The carrying amounts of the Company’s accounts payable approximate their fair values due to their short maturities.
Level 1 Assets: The Company invests in money market funds and U.S. Treasury securities. These assets are valued using observable inputs that reflect quoted prices for securities with identical characteristics.
Level 2 Assets: The Company invests in a certificate of deposit, commercial paper, and corporate debt securities. These assets are valued using observable inputs that reflect quoted prices for securities with similar characteristics and other observable inputs (such as interest rates that are observable at commonly quoted intervals).
Level 1 Liabilities: The Company values its public common stock warrants based on the market price of the warrants.
For trading securities held at the reporting date, net losses recorded during the three months ended March 31, 2025 and 2024 were immaterial
v3.25.1
INCOME TAXES
 3 Months Ended
Mar. 31, 2025
Income Tax Disclosure [Abstract]  
INCOME TAXES INCOME TAXESThe Company did not record an income tax provision for the three months ended March 31, 2025 and 2024, respectively, due to the Company’s history of losses, and accordingly, has recorded a valuation allowance against substantially all of the Company’s net deferred tax assets. The Company records a valuation allowance when it is more likely than not that some portion, or all, of the Company’s deferred tax assets will not be realized.
v3.25.1
GOVERNMENT GRANTS
 3 Months Ended
Mar. 31, 2025
Government Assistance [Abstract]  
GOVERNMENT GRANTS GOVERNMENT GRANTS
Inflation Reduction Act of 2022 (the “IRA”)
On August 16, 2022, President Biden signed the Inflation Reduction Act of 2022 into law. The IRA has significant economic incentives for both energy storage customers and manufacturers for projects placed in service after December 31, 2022. Starting in 2023, there are Production Tax Credits under Internal Revenue Code 45X (“PTC”), that can be claimed on battery components manufactured in the U.S. and sold to U.S. or foreign customers. The tax credits available to manufacturers include a credit for ten percent of the cost incurred to make electrode active materials in addition to credits of $35 per kWh of capacity of battery cells and $10 per kWh of capacity of battery modules. The credits are cumulative, meaning that companies will be able to claim each of the available tax credits based on the battery components produced and sold through 2029, after which the PTC will begin to gradually phase down through 2032, subject to pending legislation.
Since the PTC is a refundable credit (i.e., a credit with a direct-pay option available), the PTC is outside the scope of ASC 740, Income Taxes (“ASC 740”). Therefore, the Company accounts for the PTC under a government grant model. GAAP does not address the accounting for government grants received by a business entity that are outside the scope of ASC 740. The Company’s accounting policy is to analogize to IAS 20, Accounting for Government Grants and Disclosure of Government Assistance, under IFRS Accounting Standards. Under IAS 20, once it is reasonably assured that the entity will comply with the conditions of the grant, the grant money should be recognized on a systematic basis over the periods in which the entity recognizes the related expenses or losses for which the grant money is intended to compensate. The Company recognizes grants once it is probable that both of the following conditions will be met: (1) the Company is eligible to receive the grant and (2) the Company is able to comply with the relevant conditions of the grant.
The PTC is recorded as the applicable items are produced and sold. For both of the three months ended March 31, 2025 and 2024, the Company recognized net PTC benefit of $0.2 million. PTCs are recorded to cost of revenue on the condensed statements of operations and comprehensive loss. As of March 31, 2025 and December 31, 2024, grant receivable related to the PTC in the amount of $0.2 million and $1.9 million, respectively, is recorded in prepaid expenses and other current assets on the condensed balance sheets.
v3.25.1
REVENUE
 3 Months Ended
Mar. 31, 2025
Revenue from Contract with Customer [Abstract]  
REVENUE REVENUE
Disaggregated Revenue
The following table presents the Company’s revenue, disaggregated by source (in thousands):
Three Months Ended March 31,
20252024
Product revenue$407 $2,632 
Service revenue28 18 
Other revenue164 88 
Total revenue$599 $2,738 
The majority of the Company’s revenue is derived from product sales of energy storage systems.
Contract Balances
Contract assets relate to unbilled amounts resulting from contract arrangements in which the related revenue recognition performance obligations have been satisfied, however invoicing to the customer has not yet occurred. Deferred revenue (or contract liabilities) relates to consideration received from customers in advance of the Company satisfying the revenue recognition performance obligations under the related contractual arrangements. Contract balances are reported in a net contract asset or deferred revenue liability position on a contract-by-contract basis at the end of each reporting period. Contract assets are included in prepaid expenses and other current assets and deferred revenue is presented separately on the condensed balance sheets.
The following table provides information about contract assets and deferred revenue from contracts with customers (in thousands):
March 31, 2025December 31, 2024
Contract assets$547 $332 
Deferred revenue19,594 19,637 
Contract assets increased by $0.2 million during the three months ended March 31, 2025 due to the recognition of revenues for which invoicing had not yet occurred. Deferred revenue decreased by $43 thousand during the three months ended March 31, 2025 reflecting $47 thousand of revenue that was included in the deferred revenue balance at the beginning of the period offset by $4 thousand in customer advance payments.
Deferred revenue of $1.4 million is expected to be recognized within the next 12 months and non-current deferred revenue of $18.2 million is expected to be recognized thereafter as firm orders are received and fulfilled. Additionally, contracted but unsatisfied performance obligations that had not yet been billed to the customer or included in deferred revenue were $13.2 million as of March 31, 2025, for which timing of recognition is uncertain.
v3.25.1
RELATED PARTY TRANSACTIONS
 3 Months Ended
Mar. 31, 2025
Related Party Transactions [Abstract]  
RELATED PARTY TRANSACTIONS RELATED PARTY TRANSACTIONS
During the three months ended March 31, 2025, the Company recognized revenue of $28 thousand for reimbursable expenses and extended warranty services provided to related parties. During the three months ended March 31, 2024, the Company recognized revenue of $0.5 million for sale of energy storage systems and extended warranty services provided to related parties.
As of March 31, 2025, the Company had $33 thousand of deferred revenue for extended warranty services and equipment to related parties and $8 thousand of outstanding accounts receivable from related parties. As of December 31, 2024, the Company had $37 thousand of deferred revenue for extended warranty services provided to related parties and $63 thousand of outstanding accounts receivable from related parties.
As of March 31, 2025 and December 31, 2024, the Company recorded a non-refundable deposit for future equipment purchases by Honeywell of $14.4 million within non-current deferred revenue. As of March 31, 2025 and December 31, 2024, the value of the initial Performance Warrant issued to Honeywell was $0.7 million and included within other non-current assets in the condensed balance sheets. The value of the initial Performance Warrant will be recognized as an offset to revenue in the period in which revenue is earned.
v3.25.1
NET LOSS PER SHARE
 3 Months Ended
Mar. 31, 2025
Earnings Per Share [Abstract]  
NET LOSS PER SHARE NET LOSS PER SHARE
The following table presents the calculation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data):
Three Months Ended March 31,
20252024
Numerator:
Net loss attributable to common stockholders$(18,026)$(18,310)
Denominator:
Weighted-average shares outstanding – basic and diluted12,033,442 11,634,302 
Net loss per share – basic and diluted$(1.50)$(1.57)
Due to the net losses for the three months ended March 31, 2025 and 2024, basic and diluted net loss per common share were the same, as the effect of potentially dilutive securities would have been anti-dilutive.
The following outstanding balances of equivalent securities have been excluded from the calculation of diluted weighted-average common shares outstanding because the effect is anti-dilutive for the periods presented:
Three Months Ended March 31,
20252024
Stock options141,964 171,176 
RSUs718,353 1,360,256 
Warrants1,943,403 1,943,403 
Total2,803,720 3,474,835 
v3.25.1
SEGMENT AND OTHER INFORMATION
 3 Months Ended
Mar. 31, 2025
Segment Reporting [Abstract]  
SEGMENT AND OTHER INFORMATION SEGMENT AND OTHER INFORMATION
The Company has determined that its interim Chief Executive Officer is its chief operating decision maker (“CODM”). The Company operates as a single business operating segment, which includes all activities related to the design, engineering, and manufacturing of the Company’s long duration energy storage products. Accordingly, the CODM uses gross loss and net loss as reported in the statements of operations to assess financial performance and inform decisions on how to allocate resources. The financial information provided to the CODM does not contain significant disaggregated expenses outside of what is already disclosed in the statements of operations. The CODM does not evaluate results using asset or liability information.
During the three months ended March 31, 2025, one customer accounted for 94% of total revenue. During the three months ended March 31, 2024, two customers individually accounted for 75% and 19% of total revenue. The Company’s revenue is derived from U.S. and international customers. During the three months ended March 31, 2025 revenue was derived only from customers in the U.S. During the three months ended March 31, 2024, $2.1 million of total revenue was derived from customers located outside of the U.S. and the remaining $0.6 million was derived from customers in the U.S.
v3.25.1
Pay vs Performance Disclosure - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Pay vs Performance Disclosure    
Net loss $ (18,026) $ (18,310)
v3.25.1
Insider Trading Arrangements
 3 Months Ended
Mar. 31, 2025
Trading Arrangements, by Individual  
Rule 10b5-1 Arrangement Adopted false
Non-Rule 10b5-1 Arrangement Adopted false
Rule 10b5-1 Arrangement Terminated false
Non-Rule 10b5-1 Arrangement Terminated false
v3.25.1
SIGNIFICANT ACCOUNTING POLICIES (Policies)
 3 Months Ended
Mar. 31, 2025
Accounting Policies [Abstract]  
Basis of Presentation
Basis of Presentation—The accompanying condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”).
Reclassifications
Reclassifications—Certain immaterial prior year amounts have been reclassified to conform with the current year presentation. These reclassifications had no effect on the reported results of operations.
Recently Issued Accounting Pronouncements - Not Yet Adopted
Recently Issued Accounting Pronouncements—Not Yet Adopted
In November 2024, the FASB issued ASU No. 2024-03, Income Statement-Reporting Comprehensive Income (Topic 220): Expense Disaggregation Disclosures. The ASU requires entities to disaggregate operating expenses into specific categories within the notes to the financial statements to provide enhanced transparency. The ASU will be effective for fiscal years beginning after December 15, 2026, with early adoption permitted, and may be applied retrospectively or prospectively. The Company is currently evaluating the effect of this new standard on the Company's disclosures.
In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This new standard will be effective for the annual periods beginning the year ended December 31, 2025. The new standard permits early adoption and can be applied prospectively or retrospectively. We do not expect the adoption of this guidance to have a material impact on our financial statements.
v3.25.1
INVENTORY (Tables)
 3 Months Ended
Mar. 31, 2025
Inventory Disclosure [Abstract]  
Schedule of Inventory
Inventory consists of the following (in thousands):
March 31, 2025December 31, 2024
Raw materials$11,052 $12,084 
Work in process5,943 4,521 
Finished goods5,639 5,639 
Inventory, gross$22,634 $22,244 
Net realizable value adjustment(15,750)(16,603)
Inventory$6,884 $5,641 
v3.25.1
PROPERTY AND EQUIPMENT, NET (Tables)
 3 Months Ended
Mar. 31, 2025
Property, Plant and Equipment [Abstract]  
Schedule of Property and Equipment, Net
Property and equipment, net consists of the following (in thousands):
March 31, 2025December 31, 2024
Machinery and equipment$24,295 $23,306 
Leasehold improvements6,457 6,234 
Furniture and fixtures231 231 
Software614 614 
Construction in process5,911 3,722 
Total property and equipment37,508 34,107 
Less accumulated depreciation(14,998)(13,525)
Total property and equipment, net$22,510 $20,582 
v3.25.1
INTANGIBLE ASSETS, NET (Tables)
 3 Months Ended
Mar. 31, 2025
Goodwill and Intangible Assets Disclosure [Abstract]  
Schedule of Intangible Assets, Net
Intangible assets, net consisted of the following (in thousands):
March 31, 2025December 31, 2024
CostAccumulated AmortizationNet Carrying AmountCostAccumulated AmortizationNet Carrying Amount
Patents$4,990 $(401)$4,589 $4,990 $(334)$4,656 
v3.25.1
ACCRUED AND OTHER CURRENT LIABILITIES (Tables)
 3 Months Ended
Mar. 31, 2025
Payables and Accruals [Abstract]  
Schedule of Accrued and Other Current Liabilities
Accrued and other current liabilities consist of the following (in thousands):
March 31, 2025December 31, 2024
Payroll and related benefits$2,266 $4,351 
Materials and related purchases2,550 2,499 
Professional and consulting fees1,566 831 
Noncancellable purchase commitments312 233 
Environmental, Health & Safety compliance liability299 702 
Other358 699 
Total accrued and other current liabilities$7,351 $9,315 
v3.25.1
ACCRUED PRODUCT WARRANTIES (Tables)
 3 Months Ended
Mar. 31, 2025
Guarantees and Product Warranties [Abstract]  
Schedule of Product Warranty Activity
The following table summarizes product warranty activity (in thousands):
Three Months Ended March 31,
20252024
Accrued product warranties - beginning of period$3,288 $2,129 
Accruals for warranties issued 94 2,583
Repairs and replacements(1,306)(1,083)
Adjustments to existing accruals(48)(307)
Accrued product warranties - end of period$2,028 $3,322 
v3.25.1
COMMON STOCK WARRANTS (Tables)
 3 Months Ended
Mar. 31, 2025
Equity [Abstract]  
Schedule of Common Stock Warrant Activity
The following table consists of the number of shares of common stock issuable upon exercise of the respective warrants as of the dates indicated:
March 31, 2025December 31, 2024
Public Warrants764,081 764,081 
SMUD Warrant833 833 
Honeywell Warrants:
Investment Warrant708,775 708,775 
IP Warrant417,997 417,997 
Performance Warrants51,717 51,717 
Total common stock warrants1,943,4031,943,403
The table below summarizes the common stock warrant activities in the number of shares of common stock issuable upon exercise of the respective warrants during the three months ended March 31, 2025:
December 31, 2024IssuedExercisedMarch 31, 2025
Public Warrants764,081 — — 764,081
SMUD Warrant833 — — 833
Investment Warrant708,775 — — 708,775
IP Warrant417,997 — — 417,997
Performance Warrants51,717 — — 51,717
Total common stock warrants1,943,403 — — 1,943,403
The table below summarizes the common stock warrant activities in the number of shares of common stock issuable upon exercise of the respective warrants during the three months ended March 31, 2024:
December 31, 2023IssuedExercisedMarch 31, 2024
Public Warrants764,081 — — 764,081
SMUD Warrant833 — — 833 
Investment Warrant708,775 — — 708,775
IP Warrant417,997 — — 417,997
Performance Warrants51,717 — — 51,717
Total common stock warrants1,943,403 — — 1,943,403
v3.25.1
STOCK-BASED COMPENSATION (Tables)
 3 Months Ended
Mar. 31, 2025
Share-Based Payment Arrangement [Abstract]  
Schedule of Stock-Based Compensation The following table presents the amount of stock-based compensation related to stock-based awards issued to employees on the Company’s condensed statements of operations and comprehensive loss (in thousands):
Three Months Ended March 31,
20252024
Cost of revenue$620 $924 
Research and development202 401 
Sales and marketing186 95 
General and administrative226 1,434 
Total stock-based compensation$1,234 $2,854 
Schedule of Stock Options Activity
Stock option and RSU activity, prices, and values during the three months ended March 31, 2025 are as follows (in thousands, except for share, per share, and contractual term data):
Options OutstandingRSUs
Number of
shares		Weighted
average
exercise price		Weighted
average
remaining
 contractual
term
(years)		Aggregate
intrinsic
values
($'000s)		Number of plan shares outstandingWeighted average
grant date fair value
per Share
Balances as of December 31, 2024
151,288 $22.00 5.09$84 1,123,890 $27.72 
Options and RSUs granted— — 41,250 4.65 
Options exercised and RSUs released(735)5.00 (120,032)18.67 
Options and RSUs forfeited(8,589)23.97 (326,755)50.28 
Balances as of March 31, 2025
141,964 $21.97 2.89$4 718,353 $17.65 
Options vested and exercisable - December 31, 2024
130,876 $20.64 4.59$84 
Options vested and exercisable - March 31, 2025
132,644 $21.43 2.52$4 
Schedule of Restricted Stock Units Activity
Stock option and RSU activity, prices, and values during the three months ended March 31, 2025 are as follows (in thousands, except for share, per share, and contractual term data):
Options OutstandingRSUs
Number of
shares		Weighted
average
exercise price		Weighted
average
remaining
 contractual
term
(years)		Aggregate
intrinsic
values
($'000s)		Number of plan shares outstandingWeighted average
grant date fair value
per Share
Balances as of December 31, 2024
151,288 $22.00 5.09$84 1,123,890 $27.72 
Options and RSUs granted— — 41,250 4.65 
Options exercised and RSUs released(735)5.00 (120,032)18.67 
Options and RSUs forfeited(8,589)23.97 (326,755)50.28 
Balances as of March 31, 2025
141,964 $21.97 2.89$4 718,353 $17.65 
Options vested and exercisable - December 31, 2024
130,876 $20.64 4.59$84 
Options vested and exercisable - March 31, 2025
132,644 $21.43 2.52$4 
v3.25.1
FAIR VALUE MEASUREMENTS (Tables)
 3 Months Ended
Mar. 31, 2025
Fair Value Disclosures [Abstract]  
Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis
The following tables present the Company’s fair value hierarchy for its financial assets measured at fair value on a recurring basis (in thousands):
March 31, 2025
Cash Equivalents and Restricted CashShort-Term InvestmentsTotal Assets at Fair Value
Level 1:
Money market funds$5,386 $— $5,386 
U.S. Treasury securities— 3,180 3,180 
Total Level 15,386 3,180 8,566 
Level 2:
Certificate of deposit80 — 80 
Commercial paper— 1,199 1,199 
Total Level 280 1,199 1,279 
Total assets measured at fair value$5,466 $4,379 $9,845 
December 31, 2024
Cash Equivalents and Restricted CashShort-Term InvestmentsTotal Assets at Fair Value
Level 1:
Money market funds$7,232 $— $7,232 
U.S. Treasury securities— 7,142 7,142 
Total Level 17,232 7,142 14,374 
Level 2:
Certificate of deposit80 — 80 
Commercial paper4,811 7,825 12,636 
Corporate debt securities— 3,296 3,296 
Total Level 24,891 11,121 16,012 
Total assets measured at fair value$12,123 $18,263 $30,386 
The following tables present the Company’s fair value hierarchy for its financial liabilities measured at fair value on a recurring basis (in thousands):
March 31, 2025
Level 1Level 2Level 3Total
Liabilities:
Public common stock warrants917 — — 917 
Total liabilities measured at fair value$917 $— $— $917 
December 31, 2024
Level 1Level 2Level 3Total
Liabilities:
Public common stock warrants802 — — 802 
Total liabilities measured at fair value$802 $— $— $802 
v3.25.1
REVENUE (Tables)
 3 Months Ended
Mar. 31, 2025
Revenue from Contract with Customer [Abstract]  
Schedule of Revenue Disaggregated by Source
The following table presents the Company’s revenue, disaggregated by source (in thousands):
Three Months Ended March 31,
20252024
Product revenue$407 $2,632 
Service revenue28 18 
Other revenue164 88 
Total revenue$599 $2,738 
Schedule of Contract Asset and Deferred Revenue
The following table provides information about contract assets and deferred revenue from contracts with customers (in thousands):
March 31, 2025December 31, 2024
Contract assets$547 $332 
Deferred revenue19,594 19,637 
v3.25.1
NET LOSS PER SHARE (Tables)
 3 Months Ended
Mar. 31, 2025
Earnings Per Share [Abstract]  
Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Shareholders
The following table presents the calculation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data):
Three Months Ended March 31,
20252024
Numerator:
Net loss attributable to common stockholders$(18,026)$(18,310)
Denominator:
Weighted-average shares outstanding – basic and diluted12,033,442 11,634,302 
Net loss per share – basic and diluted$(1.50)$(1.57)
Schedule of Securities Excluded from Calculation of Diluted Weighted-Average Common Shares Outstanding
The following outstanding balances of equivalent securities have been excluded from the calculation of diluted weighted-average common shares outstanding because the effect is anti-dilutive for the periods presented:
Three Months Ended March 31,
20252024
Stock options141,964 171,176 
RSUs718,353 1,360,256 
Warrants1,943,403 1,943,403 
Total2,803,720 3,474,835 
v3.25.1
DESCRITPION OF BUSINESS AND BASIS OF PRESENTATION (Details)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
USD ($)
hour
Mar. 31, 2024
USD ($)
Dec. 31, 2024
USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]      
Battery, flexible energy capacity, minimum | hour 12    
Net loss $ 18,026 $ 18,310  
Net cash used in operating activities 18,238 18,910  
Cash and cash equivalents 8,422 $ 36,332 $ 13,341
Short-Term Investments 4,379   $ 18,263
Total liquid assets $ 12,800    
v3.25.1
SIGNIFICANT ACCOUNTING POLICIES (Details)
Aug. 23, 2024
$ / shares
shares
Mar. 31, 2025
$ / shares
shares
Dec. 31, 2024
$ / shares
shares
Aug. 22, 2024
shares
Accounting Policies [Abstract]        
Reverse stock split ratio, common stock 0.0667      
Common stock, shares authorized (in shares) | shares 1,000,000,000 1,000,000,000 1,000,000,000 2,000,000,000
Common stock, par value (in USD per share) | $ / shares $ 0.0001 $ 0.0001 $ 0.0001  
v3.25.1
INVENTORY - Schedule of Inventory (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Inventory Disclosure [Abstract]    
Raw materials $ 11,052 $ 12,084
Work in process 5,943 4,521
Finished goods 5,639 5,639
Inventory, gross 22,634 22,244
Net realizable value adjustment (15,750) (16,603)
Inventory $ 6,884 $ 5,641
v3.25.1
INVENTORY - Narrative (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Inventory [Line Items]    
Net realizable value adjustment $ 15,750 $ 16,603
Noncancellable Agreement    
Inventory [Line Items]    
Purchase commitments $ 300 $ 200
v3.25.1
PROPERTY AND EQUIPMENT, NET (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Property, Plant and Equipment [Line Items]      
Total property and equipment $ 37,508   $ 34,107
Less accumulated depreciation (14,998)   (13,525)
Total property and equipment, net 22,510   20,582
Depreciation expense 1,500 $ 1,200  
Machinery and equipment      
Property, Plant and Equipment [Line Items]      
Total property and equipment 24,295   23,306
Leasehold improvements      
Property, Plant and Equipment [Line Items]      
Total property and equipment 6,457   6,234
Furniture and fixtures      
Property, Plant and Equipment [Line Items]      
Total property and equipment 231   231
Software      
Property, Plant and Equipment [Line Items]      
Total property and equipment 614   614
Construction in process      
Property, Plant and Equipment [Line Items]      
Total property and equipment $ 5,911   $ 3,722
v3.25.1
INTANGIBLE ASSETS, NET - Narrative (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended
Sep. 30, 2023
Mar. 31, 2025
Acquired Finite-Lived Intangible Assets [Line Items]    
Amortization of Intangible Assets   $ 67
Patents    
Acquired Finite-Lived Intangible Assets [Line Items]    
Finite-lived intangible assets acquired $ 5,000  
Intangible asset, useful life (in years)   19 years
v3.25.1
INTANGIBLE ASSETS, NET - Schedule of Intangible Assets, Net (Details) - Patents - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Finite-Lived Intangible Assets [Line Items]    
Cost $ 4,990 $ 4,990
Accumulated Amortization (401) (334)
Net Carrying Amount $ 4,589 $ 4,656
v3.25.1
ACCRUED AND OTHER CURRENT LIABILITIES (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Payables and Accruals [Abstract]    
Payroll and related benefits $ 2,266 $ 4,351
Materials and related purchases 2,550 2,499
Professional and consulting fees 1,566 831
Noncancellable purchase commitments 312 233
Environmental, Health & Safety compliance liability 299 702
Other 358 699
Total accrued and other current liabilities $ 7,351 $ 9,315
v3.25.1
ACCRUED PRODUCT WARRANTIES (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Movement in Standard Product Warranty Accrual [Roll Forward]    
Product warranties at beginning of period $ 3,288 $ 2,129
Accruals for warranties issued 94 2,583
Repairs and replacements (1,306) (1,083)
Adjustments to existing accruals (48) (307)
Product warranties at end of period $ 2,028 $ 3,322
v3.25.1
COMMITMENTS AND CONTINGENCIES (Details)
 1 Months Ended 3 Months Ended
Sep. 30, 2023
USD ($)
Mar. 31, 2025
USD ($)
Mar. 31, 2024
USD ($)
Dec. 31, 2024
USD ($)
Nov. 01, 2024
USD ($)
production_line
Other Commitments [Line Items]          
Restricted certificate of deposit   $ 75,000      
Research and development   2,478,000 $ 3,546,000    
Joint Development Agreement | UOP          
Other Commitments [Line Items]          
Research and development $ 8,000,000 0      
Noncancellable Agreement          
Other Commitments [Line Items]          
Purchase commitments   300,000   $ 200,000  
Cancellable Agreement          
Other Commitments [Line Items]          
Purchase commitments   4,800,000   6,000,000.0  
Letter of Credit | First Republic Bank          
Other Commitments [Line Items]          
Standby letter of credit       75,000  
Draws against letter of credit   0 0    
Letter of Credit | CitiBank N.A          
Other Commitments [Line Items]          
Standby letter of credit       600,000  
Draws against letter of credit   0 0    
Letter of Credit | CitiBank N.A | Asset Pledged as Collateral          
Other Commitments [Line Items]          
Amounts pledged as collateral   600,000      
Letter of Credit | First Citizens BancShares          
Other Commitments [Line Items]          
Standby letter of credit       $ 200,000  
Draws against letter of credit   0 $ 0    
Letter of Credit | First Citizens BancShares | Asset Pledged as Collateral          
Other Commitments [Line Items]          
Amounts pledged as collateral   200,000      
Secured Debt | Credit Agreement | Line of Credit          
Other Commitments [Line Items]          
Standby letter of credit         $ 22,700,000
Number of production lines financed | production_line         2,000
Maximum borrowing capacity         $ 20,000,000.0
Outstanding notes payable   $ 0      
v3.25.1
COMMON STOCK WARRANTS - Schedule of Common Stock Warrant Balances (Details) - shares
Mar. 31, 2025
Dec. 31, 2024
Mar. 31, 2024
Dec. 31, 2023
Class of Warrant or Right [Line Items]        
Warrants (in shares) 1,943,403 1,943,403 1,943,403 1,943,403
Public Warrants        
Class of Warrant or Right [Line Items]        
Warrants (in shares) 764,081 764,081 764,081 764,081
SMUD Warrant        
Class of Warrant or Right [Line Items]        
Warrants (in shares) 833 833 833 833
Investment Warrant        
Class of Warrant or Right [Line Items]        
Warrants (in shares) 708,775 708,775 708,775 708,775
IP Warrant        
Class of Warrant or Right [Line Items]        
Warrants (in shares) 417,997 417,997 417,997 417,997
Performance Warrants        
Class of Warrant or Right [Line Items]        
Warrants (in shares) 51,717 51,717 51,717 51,717
v3.25.1
COMMON STOCK WARRANTS - Narrative (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended
Sep. 21, 2023
USD ($)
tradingDay
$ / shares
shares
Oct. 08, 2021
$ / shares
shares
Sep. 21, 2020
shares
Mar. 31, 2025
USD ($)
$ / shares
shares
Mar. 31, 2024
USD ($)
shares
Dec. 31, 2024
shares
Dec. 31, 2023
shares
Sep. 16, 2022
$ / shares
shares
Nov. 09, 2021
shares
Class of Warrant or Right [Line Items]                  
Warrants issued (in shares)       0 0        
Warrants (in shares)       1,943,403 1,943,403 1,943,403 1,943,403    
Conversion ratio                 1
Change in fair value of common stock warrant liabilities | $       $ 115 $ 0        
Number of shares with vesting conditions met (in shares)       833   833      
Scenario two                  
Class of Warrant or Right [Line Items]                  
Price of common stock triggering redemption of warrants (in USD per share) | $ / shares       $ 150.00          
Trading days triggering redemption of warrants       20 days          
Consecutive trading day period       30 days          
STWO | Common Stock                  
Class of Warrant or Right [Line Items]                  
Price per share (in USD per share) | $ / shares   $ 172.50              
STWO | Private warrants | ACON S2 Acquisition Corp. Sponsor | Private Placement                  
Class of Warrant or Right [Line Items]                  
Shares issued (in shares)     4,666,667            
STWO | Public Warrant Holders | Common Stock                  
Class of Warrant or Right [Line Items]                  
Issuance of shares (in shares)   1              
Warrants (in shares)   15              
Public common stock warrants                  
Class of Warrant or Right [Line Items]                  
Warrants issued (in shares)       0 0        
Warrants (in shares)       764,081 764,081 764,081 764,081    
Public common stock warrants | Scenario one                  
Class of Warrant or Right [Line Items]                  
Redemption price per share (in USD per share) | $ / shares       $ 0.01          
Price of common stock triggering redemption of warrants (in USD per share) | $ / shares       $ 270.00          
Trading days triggering redemption of warrants       20 days          
Consecutive trading day period       30 days          
Public common stock warrants | Scenario two                  
Class of Warrant or Right [Line Items]                  
Redemption price per share (in USD per share) | $ / shares       $ 0.10          
Public common stock warrants | STWO                  
Class of Warrant or Right [Line Items]                  
Warrants issued (in shares)   8,333,287              
Warrants term (in years)   5 years              
Private warrants                  
Class of Warrant or Right [Line Items]                  
Warrants (in shares)   4,083,334              
Warrants vested (in shares)   3,500,000              
Private warrants | ACON S2 Acquisition Corp. Sponsor                  
Class of Warrant or Right [Line Items]                  
Warrants forfeited (in shares)   583,333              
Earnout warrants                  
Class of Warrant or Right [Line Items]                  
Warrants vested (in shares)                 583,334
SMUD Warrant                  
Class of Warrant or Right [Line Items]                  
Warrants issued (in shares)       0 0        
Warrants (in shares)       833 833 833 833    
Class of warrant or right, number of securities called by each warrant or right (in shares)               33,333  
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares               $ 64.44  
Honeywell warrants                  
Class of Warrant or Right [Line Items]                  
Shares issued (in shares) 1,099,450                
Proceeds from Issuance of Warrants | $ $ 27,500                
Honeywell warrants | IP Warrant                  
Class of Warrant or Right [Line Items]                  
Warrants (in shares) 417,997                
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares $ 43.50                
Honeywell warrants | Investment Warrant                  
Class of Warrant or Right [Line Items]                  
Warrants (in shares) 708,775                
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares $ 28.35                
Performance warrant                  
Class of Warrant or Right [Line Items]                  
Warrants term (in years) 5 years                
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares $ 21.75                
Proceeds from Issuance of Warrants | $ $ 300,000                
Additional performance warrants | $ $ 15,000                
Number of trading days | tradingDay 15                
Performance warrant | UOP LLC | Related party                  
Class of Warrant or Right [Line Items]                  
Warrants term (in years) 5 years                
Class of warrant or right, number of securities called by each warrant or right (in shares) 51,717                
Exchange for pre-payment of equipment | $ $ 15,000                
v3.25.1
COMMON STOCK WARRANTS - Schedule of Common Stock Warrant Activity (Details) - shares
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Class of Warrant or Right [Roll Forward]    
Warrants or rights at beginning of period (in shares) 1,943,403 1,943,403
Warrants issued (in shares) 0 0
Warrants exercised (in shares) 0 0
Warrants or rights at end of period (in shares) 1,943,403 1,943,403
Public Warrants    
Class of Warrant or Right [Roll Forward]    
Warrants or rights at beginning of period (in shares) 764,081 764,081
Warrants issued (in shares) 0 0
Warrants exercised (in shares) 0 0
Warrants or rights at end of period (in shares) 764,081 764,081
SMUD Warrant    
Class of Warrant or Right [Roll Forward]    
Warrants or rights at beginning of period (in shares) 833 833
Warrants issued (in shares) 0 0
Warrants exercised (in shares) 0 0
Warrants or rights at end of period (in shares) 833 833
Investment Warrant    
Class of Warrant or Right [Roll Forward]    
Warrants or rights at beginning of period (in shares) 708,775 708,775
Warrants issued (in shares) 0 0
Warrants exercised (in shares) 0 0
Warrants or rights at end of period (in shares) 708,775 708,775
IP Warrant    
Class of Warrant or Right [Roll Forward]    
Warrants or rights at beginning of period (in shares) 417,997 417,997
Warrants issued (in shares) 0 0
Warrants exercised (in shares) 0 0
Warrants or rights at end of period (in shares) 417,997 417,997
Performance Warrants    
Class of Warrant or Right [Roll Forward]    
Warrants or rights at beginning of period (in shares) 51,717 51,717
Warrants issued (in shares) 0 0
Warrants exercised (in shares) 0 0
Warrants or rights at end of period (in shares) 51,717 51,717
v3.25.1
STOCK-BASED COMPENSATION - Schedule of Stock-Based Compensation (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]    
Total stock-based compensation $ 1,234 $ 2,854
General and administrative    
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]    
Total stock-based compensation 226 1,434
Cost of revenue    
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]    
Total stock-based compensation 620 924
Research and development    
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]    
Total stock-based compensation 202 401
Sales and marketing    
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]    
Total stock-based compensation $ 186 $ 95
v3.25.1
STOCK-BASED COMPENSATION - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended
Jan. 01, 2022
Mar. 31, 2025
Mar. 31, 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of potential increase of shares authorized   5.00%  
Award vesting period   3 years  
Unamortized stock-based compensation costs   $ 11,100  
Weighted average period of recognition for unamortized stock-based compensation costs   2 years 1 month 13 days  
Total stock-based compensation   $ 1,234 $ 2,854
Tranche one      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage   25.00%  
Tranche two      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage   6.25%  
2021 Equity Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Potential increase of shares authorized (in shares)   1,017,333  
Increase in shares authorized for issuance (in shares) 599,325    
Shares authorized for issuance (in shares)   2,353,325  
Shares available for future grant (in shares)   1,223,746,000  
Stock options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares available for grant   $ 97,834  
Stock options | 2021 Equity Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period   4 years  
Share expiration period   10 years  
ESPP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum employee stock purchase amount   $ 25  
ESPP purchase price of common stock, percent of fair market value   85.00%  
Total stock-based compensation   $ 51 $ 84
v3.25.1
STOCK-BASED COMPENSATION - Schedule of Share-based Payment Arrangement, Option and RSU Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Options Outstanding      
Options outstanding at beginning of period (in shares) 151,288    
Options granted (in shares) 0 0  
Options released (in shares) (735)    
Options forfeited (in shares) (8,589)    
Options outstanding at end of period (in shares) 141,964   151,288
Options vested and exercisable (in shares) 132,644   130,876
Weighted average exercise price      
Weighted average exercise price of options outstanding at beginning of period (in USD per share) $ 22.00    
Weighted average exercise price of options granted (in USD per share) 0    
Weighted average exercise price of options released (in USD per share) 5.00    
Weighted average exercise price of options forfeited (in USD per share) 23.97    
Weighted average exercise price of options outstanding at end of period (in USD per share) 21.97   $ 22.00
Weighted average exercise price of options vested and exercisable (in USD per share) $ 21.43   $ 20.64
Options, Additional Disclosures      
Weighted average remaining contractual term of options outstanding 2 years 10 months 20 days   5 years 1 month 2 days
Weighted average remaining contractual term of options vested and exercisable 2 years 6 months 7 days   4 years 7 months 2 days
Aggregate intrinsic value of options outstanding $ 4   $ 84
Aggregate intrinsic value of options vested and exercisable $ 4   $ 84
RSUs      
RSUs      
Units outstanding at beginning of period (in shares) 1,123,890    
Units granted (in shares) 41,250    
Units released (in shares) (120,032)    
Units forfeited (in shares) (326,755)    
Units outstanding at end of period (in shares) 718,353   1,123,890
Weighted average grant date fair value per Share      
Weighted average grant date fair value per share of units outstanding at beginning of period (in USD per share) $ 27.72    
Weighted average grant date fair value per share of units granted (in USD per share) 4.65    
Weighted average grant date fair value per share of units released (in USD per share) 18.67    
Weighted average grant date fair value per share of units forfeited (in USD per share) 50.28    
Weighted average grant date fair value per share of units outstanding at end of period (in USD per share) $ 17.65   $ 27.72
v3.25.1
FAIR VALUE MEASUREMENTS - Schedule of Assets and Liabilities on Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Assets, Fair Value Disclosure [Abstract]    
Short-Term Investments $ 4,379 $ 18,263
Recurring    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash 5,466 12,123
Short-Term Investments 4,379 18,263
Total Assets at Fair Value 9,845 30,386
Liabilities, Fair Value Disclosure [Abstract]    
Total liabilities measured at fair value 917 802
Recurring | Public common stock warrants    
Liabilities, Fair Value Disclosure [Abstract]    
Public common stock warrants 917 802
Recurring | Level 1    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash 5,386 7,232
Short-Term Investments 3,180 7,142
Total Assets at Fair Value 8,566 14,374
Liabilities, Fair Value Disclosure [Abstract]    
Total liabilities measured at fair value 917 802
Recurring | Level 1 | Public common stock warrants    
Liabilities, Fair Value Disclosure [Abstract]    
Public common stock warrants 917 802
Recurring | Level 1 | Money market funds    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash 5,386 7,232
Short-Term Investments 0 0
Total Assets at Fair Value 5,386 7,232
Recurring | Level 1 | U.S. Treasury securities    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash 0 0
Short-Term Investments 3,180 7,142
Total Assets at Fair Value 3,180 7,142
Recurring | Level 2    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash 80 4,891
Short-Term Investments 1,199 11,121
Total Assets at Fair Value 1,279 16,012
Liabilities, Fair Value Disclosure [Abstract]    
Total liabilities measured at fair value 0 0
Recurring | Level 2 | Public common stock warrants    
Liabilities, Fair Value Disclosure [Abstract]    
Public common stock warrants 0 0
Recurring | Level 2 | Certificate of deposit    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash 80 80
Short-Term Investments 0 0
Total Assets at Fair Value 80 80
Recurring | Level 2 | Commercial paper    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash 0 4,811
Short-Term Investments 1,199 7,825
Total Assets at Fair Value 1,199 12,636
Recurring | Level 2 | Corporate debt securities    
Assets, Fair Value Disclosure [Abstract]    
Cash Equivalents and Restricted Cash   0
Short-Term Investments   3,296
Total Assets at Fair Value   3,296
Recurring | Level 3    
Liabilities, Fair Value Disclosure [Abstract]    
Total liabilities measured at fair value 0 0
Recurring | Level 3 | Public common stock warrants    
Liabilities, Fair Value Disclosure [Abstract]    
Public common stock warrants $ 0 $ 0
v3.25.1
INCOME TAXES (Details) - USD ($)
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Income Tax Disclosure [Abstract]    
Income tax expense provision $ 0 $ 0
v3.25.1
GOVERNMENT GRANTS (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Government Assistance [Line Items]      
Government Assistance, Current, Statement of Financial Position [Extensible Enumeration] Prepaid expenses and other current assets    
Production Tax Credits      
Government Assistance [Line Items]      
Government assistance amount $ 200 $ 200  
Current grants receivable $ 200   $ 1,900
v3.25.1
REVENUE - Schedule of Revenue Disaggregated by Source (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Disaggregation of Revenue [Line Items]    
Revenue $ 599 $ 2,738
Product revenue    
Disaggregation of Revenue [Line Items]    
Revenue 407 2,632
Service revenue    
Disaggregation of Revenue [Line Items]    
Revenue 28 18
Other revenue    
Disaggregation of Revenue [Line Items]    
Revenue $ 164 $ 88
v3.25.1
REVENUE - Schedule of Revenue Contract Assets and Deferred Revenue (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Revenue from Contract with Customer [Abstract]    
Contract assets $ 547 $ 332
Deferred revenue $ 19,594 $ 19,637
v3.25.1
REVENUE - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Dec. 31, 2024
Revenue from Contract with Customer [Abstract]    
Contract assets increased $ 200  
Increase in deferred revenue 43  
Revenue recognized 47  
Advance payment 4  
Deferred revenue, current 1,359 $ 5,237
Deferred revenue, non-current 18,200  
Unbilled contracts receivable $ 13,200  
v3.25.1
RELATED PARTY TRANSACTIONS (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Related Party Transaction [Line Items]      
Revenue $ 599 $ 2,738  
Deferred revenue, current 1,359   $ 5,237
Accounts receivable, net 253   215
Related party      
Related Party Transaction [Line Items]      
Revenue 28 524  
Energy Warehouse Sales | Related party      
Related Party Transaction [Line Items]      
Revenue 28    
Deferred revenue, current 33   37
Accounts receivable, net 8   63
Energy Storage Systems Sales | Related party      
Related Party Transaction [Line Items]      
Revenue   $ 500  
Honeywell ACS Ventures LLC | Related party      
Related Party Transaction [Line Items]      
Prepayment of equipment 14,400   14,400
Deferred revenue, non-current - related parties $ 700   $ 700
v3.25.1
NET LOSS PER SHARE - Schedule of Basic and Diluted Net Loss Per Share (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Numerator:    
Net loss to common stockholders, basic $ (18,026) $ (18,310)
Net loss to common stockholders, diluted $ (18,026) $ (18,310)
Denominator:    
Weighted-average shares outstanding – basic (in shares) 12,033,442 11,634,302
Weighted-average shares outstanding – diluted (in shares) 12,033,442 11,634,302
Net loss per share - basic (in USD per share) $ (1.50) $ (1.57)
Net loss per share - diluted (in USD per share) $ (1.50) $ (1.57)
v3.25.1
NET LOSS PER SHARE - Schedule Antidilutive Securities (Details) - shares
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Number of securities excluded (in shares) 2,803,720 3,474,835
Stock options    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Number of securities excluded (in shares) 141,964 171,176
RSUs    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Number of securities excluded (in shares) 718,353 1,360,256
Warrants    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Number of securities excluded (in shares) 1,943,403 1,943,403
v3.25.1
SEGMENT AND OTHER INFORMATION - Narrative (Details)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
USD ($)
segment
Mar. 31, 2024
USD ($)
Segment Reporting Information [Line Items]    
Number of operating segments | segment 1  
Revenue $ 599 $ 2,738
Non-US    
Segment Reporting Information [Line Items]    
Revenue   2,100
UNITED STATES    
Segment Reporting Information [Line Items]    
Revenue   $ 600
Customer One | Revenue benchmark | Customer concentration risk    
Segment Reporting Information [Line Items]    
Concentration risk, percentage 94.00% 75.00%
Customer Two | Revenue benchmark | Customer concentration risk    
Segment Reporting Information [Line Items]    
Concentration risk, percentage   19.00%