Item 8.01. Other Events. On February 18, 2025, Tecogen Inc. (“ Company,” “we,” “our, ”us”) entered into an amendment (the “ Amendment”) to its promissory notes to John N. Hatsopoulos, our Lead Director and a shareholder of the Company, consisting of a $500,000 promissory note dated October 10, 2023, as amended March 21, 2024, and a $500,000 promissory note dated July 23, 2024 (collectively the “ Promissory Notes”). The Amendment extended the maturity date of the Promissory Notes to July 31, 2026, and permits Mr. Hatsopoulos to receive payment of the balance of the Promissory Notes in cash or in common stock of the Company. If requested by Mr. Hatsopoulos, the balance of one or both of the Promissory Notes will be converted to the number of shares of the Company’s common stock determined by dividing the outstanding principal and accrued interest by the average closing price per share for the Company’s common stock during the thirty day period prior to the date of conversion of the loan balance to common stock. The foregoing description of the Amendment is not complete and is qualified in its entirety by reference to the full text of the Amendment, a copy of which is filed as Exhibit 99.1 to this Current Report on Form 8-K.

Item 9.01. Financial Statements and Exhibits. (d) Exhibits Exhibit Description ─────────────────────────────────────────────────────────── 99.01 Amendment to Tecogen Inc. Promissory Notes SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the Company has duly caused this report to be signed on its behalf by the undersigned, hereunto duly authorized. TECOGEN INC. By: /s/ Abinand Rangesh February 19, 2025 Abinand Rangesh, Chief Executive Officer