Item 8.01 Other Events On September 2, 2025, in accordance with its Policy and Procedures Governing Related Person Transactions, the Audit Committee of Tecogen Inc., a Delaware corporation(“ Company”) approved the prepayment by the Company of two promissory notes in favor of John N. Hatsopoulos, a director of the Company, in the aggregate principal amount of $1 million, together with interest accrued thereon through the dates the promissory notes are repaid. The promissory notes to be prepaid consist of: (i) a promissory note dated October 10, 2023 in the principal amount of $500,000 with interest accruing at an annual rate of 5.12%, as amended on March 31, 2024 and on February 18, 2025 (the “ October 2023 Note”), and (ii) a promissory note dated July 23, 2024 in the principal amount of $500,000 with interest accruing at an annual rate of 5.06%, as amended on February 18, 2025 (the “ July 2024 Note”). As amended, both promissory notes have a maturity date of July 31, 2026. The October 2024 Note was prepaid on September 3, 2025 and the July 2024 Note was prepaid on September 4, 2025. The aggregate payments pursuant to the promissory notes was $1,076,955.62, including aggregate interest in the amount of $76,955.62. Repayment of the promissory notes eliminated the balance of the Company’s outstanding debt, as well as approximately $46,159 of interest charges that would have been payable with respect to the promissory notes between the repayment dates and the maturity dates thereof. 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 September 4, 2025 Abinand Rangesh, Chief Executive Officer