Exhibit 5.1

Wilson Sonsini Goodrich & Rosati Professional Corporation   650 Page Mill Road Palo Alto, California 94304-1050   O: 650.493.9300 F: 650.493.6811

December 9, 2021

ESS Tech, Inc.

132426440 SW Parkway Ave. Bldg. 83

Wilsonville, OR 97070

Re: Registration Statement on Form S-1 (File No. 333-260693)

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-1 (File No. 333-260693) filed by ESS Tech, Inc. (f/k/a ACON S2 Acquisition Corp.), a Delaware corporation (the “Company”), with the Securities and Exchange Commission (the “Commission”), as amended by Post-Effective Amendment No. 1 being filed with the Commission on the date hereof (such registration statement as so amended, the “Registration Statement”), in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), of up to 125,952,180 shares of the Company’s Common Stock, par value $0.0001 per share (the “Securities”).

The Securities offered pursuant to the Registration Statement include (i) an aggregate of 111,489,068 outstanding shares of the Company’s Common Stock (the “Outstanding Shares”) to be sold by selling securityholders named in the Registration Statement), (ii) an aggregate of up to 13,638,114 shares of Common Stock (the “Earnout Shares”) issuable upon the achievement of certain trading price targets in accordance with the Merger Agreement (as defined in the Registration Statement), and (iii) an aggregate of up to 824,998 shares of the Company’s Common Stock (“Equity Award Shares”) issuable upon the vesting of restricted stock units.

We are acting as counsel for the Company in connection with the registration of the Securities. As such counsel, we have made such legal and factual examinations and inquiries as we have deemed necessary or advisable for the purpose of rendering the opinions and statements set forth below. In rendering the opinions and statements expressed below, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary for the purposes of rendering this opinion.

In addition, we have reviewed originals or copies of such corporate records of the Company, certificates of public officials, a certificate of an officer of the Company as to factual matters, and such other documents which we consider necessary or advisable for the purpose of rendering the opinions set forth below. We have not independently established the facts stated therein.

AUSTIN    BEIJING    BOSTON    BRUSSELS    HONG KONG    LONDON    LOS ANGELES    NEW YORK    PALO ALTO

SAN DIEGO    SAN FRANCISCO    SEATTLE    SHANGHAI    WASHINGTON, DC    WILMINGTON, DE

ESS Tech, Inc.

December 9, 2021

Page 2

In our examination, we have assumed the genuineness of all signatures, the authenticity and completeness of all documents submitted to us as originals, the conformity with the originals of all documents submitted to us as copies, the authenticity of the originals of such documents and the legal competence of all signatories to such documents. We have also assumed the authority of such persons signing on behalf of the parties thereto other than the Company and the due authorization, execution and delivery of all documents by the parties thereto other than the Company. We have assumed that the certificates representing the Securities have been properly authenticated by the signature of an authorized officer of the Company’s transfer agent. We have also assumed the conformity of the documents filed with the Commission via the Electronic Data Gathering, Analysis and Retrieval System (“EDGAR”), except for required EDGAR formatting changes, to physical copies submitted for our examination and the absence of any evidence extrinsic to the provisions of the written agreements between the parties that the parties intended a meaning contrary to that expressed by those provisions.

We express no opinion as to any matter relating to the laws of any jurisdiction other than the federal laws of the United States of America and the General Corporation Law of the State of Delaware.

Based upon and subject to the foregoing qualifications, assumptions and limitations and the further limitations set out below, we are of the opinion that:

1. With respect to the Outstanding Shares to be offered pursuant to the Registration Statement, such Outstanding Shares have been duly authorized and are validly issued, fully paid and nonassessable;

2. With respect to the Earnout Shares to be offered pursuant to the Registration Statement, when such shares are issued in accordance with the terms of the Merger Agreement, such Earnout Shares will have been validly issued, fully paid and nonassessable; and

3. With respect to the Equity Award Shares to be offered pursuant to the Registration Statement, when such shares are issued upon the vesting of such equity awards, such Equity Award Shares will have been validly issued, fully-paid and nonassessable.

Our opinion that any document is legal, valid and binding is qualified as to:

(a) limitations imposed by bankruptcy, insolvency, reorganization, arrangement, fraudulent transfer, moratorium or other similar laws relating to or affecting the rights of creditors generally;

(b) rights to indemnification and contribution, which may be limited by applicable law or equitable principles; and

(c) the effect of general principles of equity, including without limitation concepts of materiality, reasonableness, good faith and fair dealing, and the possible unavailability of specific performance or injunctive relief, whether considered in a proceeding in equity or at law.

ESS Tech, Inc.

December 9, 2021

Page 3

This opinion is furnished to you in connection with the filing of the Registration Statement, and is not to be used, circulated, quoted or otherwise relied upon for any other purpose.

We hereby consent to the filing of this opinion as an exhibit to the above-referenced Registration Statement and to the use of our name wherever it appears in the Registration Statement, the Prospectus, any Prospectus Supplement, and in any amendment or supplement thereto. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Wilson Sonsini Goodrich & Rosati

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation