ESS Tech Inc美股招股说明书(2025-12-11版)

We have entered into a Sales Agreement, dated November13, 2025, (the “Sales Agreement”), with Yorkville Securities, LLC(“Yorkville Securities”), BMO Capital Markets Corp. (“BMO”), Canaccord Genuity LLC (“Canaccord”), Needham & Company, LLC(“Needham”) and Stifel, Nicolaus & Company, Incorporated (“Stifel”) (each of Yorkville Securities, BMO, Canaccord, Needham andStifel individually an “Agent” and collectively, the “Agents”) relating to shares of our common stock, $0.0001 par value per share,offered by this prospectus supplement and the accompanying prospectus. In accordance with the terms of the Sales Agreement, wemay offer and sell shares of our common stock under this prospectus supplement having an aggregate gross sales price of up to$75,000,000 shares of the Company’s common stock, from time to time through or to the Agents, as sales agents or principals. Our common stock is listed on the New York Stock Exchange (the “NYSE”) under the symbol “GWH.” On December 10, 2025,the last quoted sale price for our common stock as reported on the NYSE was $2.31 per share. Sales of shares of our common stock under this prospectus supplement, if any, may be made by any method deemed to be an “atthe market offering” as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended (the “Securities Act”).The Agents are not required to sell any specific number of shares of our common stock. The Agents have agreed to use theircommercially reasonable efforts consistent with its normal trading and sales practices, on mutually agreed terms between the Agentsand us. There is no arrangement for funds to be received in any escrow, trust or similar arrangement. The Agents will be entitled to compensation under the terms of the Sales Agreement at a commission rate equal to 3.0% of thegross proceeds of the sales price of common stock that they sell under the Sales Agreement. The net proceeds from any sales under thisprospectus supplement will be used as described under “Use of Proceeds.” The proceeds we receive from sales of our common stock,if any, will depend on the number of shares actually sold and the offering price of such shares. In connection with the sale of our shares of common stock on our behalf, the Agents will each be deemed to be an “underwriter”within the meaning of the Securities Act, and their compensation as Agents will be deemed to be underwriting commissions ordiscounts. We have also agreed to provide indemnification and contribution to the Agents with respect to certain liabilities, includingliabilities under the Securities Act. See the section titled “Plan of Distribution (Conflict of Interest)” beginning on pageS-11of thisprospectus supplement. Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on pageS-3of this prospectussupplement, as well as the documents incorporated by reference in this prospectus supplement, for a discussion of the factorsyou should carefully consider before deciding to purchase our securities. Neither the Securities and Exchange Commission(“SEC”) nor any state securities commission has approved or disapproved of these securities or determined if this prospectussupplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense. Yorkville SecuritiesCanaccord GenuityNeedham & Company Stifel The date of this prospectus supplement is December 11, 2025. TABLE OF CONTENTS Prospectus Supplement About This Prospectus SupplementProspectus Supplement SummaryRisk FactorsSpecial Note Regarding Forward-Looking StatementsUse of ProceedsDescription of Capital StockDilutionPlan of Distribution (Conflict of Interest)Legal MattersExpertsWhere You Can Find Additional InformationIncorporation of Certain Information by Reference Prospectus About this ProspectusProspectus SummaryRisk FactorsForward-Looking StatementsUse of ProceedsDescription of Capital StockDescription of Debt SecuritiesDescriptionof Depositary SharesDescription of WarrantsDescription of Subscription RightsDescription of Purchase ContractsDescription of UnitsPlan of DistributionLegal MattersExpertsWhere You Can Find More InformationIncorporation by Reference ABOUT THIS PROSPECTUS SUPPLEMENT This document is in two parts. The first part is this prospectus supplement, including the documents incorporated by reference,which describes the specific terms of this offering. The second part, the accompanying prospectus, including the documentsincorporated by reference, provides more general information. Before you invest in our securities, you should read both this prospectussupplement and the accompanying prospectus together with the additional information described in the sections titled “Where You CanFind More Information” and “Incorporation of Certain Information by Reference.” Neither we nor any Agent has authorized anyone to provide you with information that is different from that contained, orincorporated by r