Eos Energy launches $150M rights offering at $5.48/unit to fund Frontier Power investment

On July 2, 2026, Eos Energy Enterprises, Inc. (the “Company”) issued a press release announcing the commencement of its previously-announced rights offering pursuant to a shelf registration statement filed on Form S-3 (File No. 333-295819) (the “Registration Statement”) with the Securities and Exchange Commission (“SEC”) on May 13, 2026, and the prospectus supplement relating to the rights offering filed with the SEC on July 2, 2026

the Company is filing certain ancillary documents as Exhibits 4.1, 4.2, 4.3, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, and 99.7 to this Current Report on Form 8-K for the purpose of incorporating such items by reference to the Registration Statement, of which the Prospectus forms a part. The Company is also filing as Exhibit 5.1 the opinion of Davis Polk & Wardwell LLP in connection with the issuance of the subscription rights and the Company’s common stock, par value $0.0001 per share (“Common Stock”), and warrants to purchase shares of Common Stock, issuable upon exercise of such subscription rights.

A copy of the Prospectus may also be obtained by contacting the information agent for the rights offering, Sodali & Co., at (203) 658-9400 (banks and brokers), (833) 225-0490 (individuals call toll-free) or EOSE.info@investor.sodali.com.

The Rights Offering will expire at 5:00 p.m., New York City time, on July 21, 2026, unless extended as described in the Prospectus Supplement (the "Expiration Time"). ... Expected commencement of trading for the Rights on the Nasdaq Capital Market under the symbol "EOSER." July 6, 2026

Conditions to Completion of the Rights Offering. The completion of the Rights Offering is conditional on the execution of the JV Agreement and the execution of certain commercial framework guidelines by the parties to the JV Transaction. We may terminate the Rights Offering, in whole or in part, if at any time before completion of the Rights Offering there is any judgment, order, decree, injunction, statute, law or regulation entered, enacted, amended or held to be applicable to the Rights Offering that in the sole judgment of our board of directors would or might make the Rights Offering or its completion, whether in whole or in part, illegal or otherwise restrict or prohibit completion of the Rights Offering.

The Company has the right to terminate the Rights Offering for any reason before the Rights expire.

As described in the Prospectus, Record Date Holders will receive a Right for each share of Common Stock and each Participating Warrant held by such holder as of the Record Date. The total number of Rights to be issued to each Record Date Holder was rounded down to the nearest whole number and the subscription agent instructed, or instructed DTC to instruct, all brokers, dealers, trustees and depositaries for securities or any other agents who hold shares of Common Stock or Participating Warrants for the account of others to effect such rounding with respect to each beneficial holder. Each whole Right entitles a Rights Holder to purchase 0.071193 of a Unit, which is referred to as the “Basic Subscription Rights.”

If any Units available for purchase in the Rights Offering are not subscribed for by Rights Holders pursuant to the Basic Subscription Rights (the “Remaining Units”), a Rights Holder that has exercised fully its Rights pursuant to the Basic Subscription Rights may subscribe for any Remaining Units that are not otherwise subscribed for by Rights Holders, on the terms and subject to the conditions set forth in the Prospectus, including as to proration. We refer to this Over-subscription privilege as the “Over-subscription Privilege.”

As described in the Prospectus Supplement, Record Date Holders will receive a Right for each share of Common Stock and each Participating Warrant held by such holder as of the Record Date. The total number of Rights to be issued to each Record Date Holder was rounded down to the nearest whole number and the subscription agent instructed, or instructed DTC to instruct, all brokers, dealers, trustees and depositaries for securities or any other agents who hold shares of Common Stock or Participating Warrants for the account of others to effect such rounding with respect to each beneficial holder. Each whole Right entitles a Rights Holder to purchase 0.071193 of a Unit, which is referred to as the “Basic Subscription Rights.”

If any Units available for purchase in the Rights Offering are not subscribed for by Rights Holders pursuant to the Basic Subscription Rights (the “Remaining Units”), a Rights Holder that has exercised fully its Rights pursuant to the Basic Subscription Rights may subscribe for any Remaining Units that are not otherwise subscribed for by Rights Holders, on the terms and subject to the conditions set forth in the Prospectus Supplement, including as to proration.

The Rights may be exercised by the holders thereof (the “Rights Holders”) at any time during the subscription period, which commences on July 2, 2026. The Rights Offering will expire at 5:00 p.m., New York City time, on July 21, 2026, unless extended by the Company in its sole discretion (as it may be extended, the “Expiration Time”). The Rights are transferable and are expected to be listed for trading on The Nasdaq Capital Market under the symbol “EOSER” beginning on July 6, 2026 until the Expiration Time.

prospectus supplement dated July 2, 2026

$5.481

Pursuant to the rights offering, the Company will distribute Rights to acquire an aggregate of 27,367,171 units (the “Units”) on July 2, 2026 (the “Distribution Date”) at a price per Unit of $5.481 to the Eligible Holders as of the Record Date. Each Unit consists of one share of the Company’s common stock and 0.4388 of a warrant to purchase one share of the Company’s common stock at an exercise price of $5.481 per whole share.

The Company intends to use the net proceeds of the Rights Offering, if any, to fund its previously announced investment in Frontier Power USA Parent, LLC (“Frontier”).

The Company has applied to have the Rights admitted to trading on the Nasdaq Capital Market, where it expects them to begin trading under the symbol “EOSER” on July 6, 2026. The Company has also applied to have the Warrants admitted to trading on the Nasdaq Capital Market under the symbol “EOSEW”. However, no assurance can be given that such listing application will be approved.