Mainspring Energy builds modular linear generators that produce electricity at customer sites for utilities, data centers and industrial operators. Axios reports the company closed a $242 million Series G, which the publication said could be Mainspring’s final private financing before an IPO.
Mainspring describes a unit as turning back-and-forth motion directly into electricity. A low-temperature, non-combustion reaction drives oscillators through copper coils, while power electronics and software regulate the cycle. The company says the same hardware can switch among natural gas, biogas, propane and hydrogen.
The company sells standardized 2.5-megawatt configurations built from ten 0.25-megawatt generators. Mainspring says the modules can ramp from zero to full output and run independently or alongside the grid, letting a customer add onsite capacity without building around a single large machine.
A sales agreement announced the same day puts that modular pitch into a utility order. American Electric Power purchased an initial 190 megawatts for delivery beginning in 2027; the broader agreement allows it to buy up to 1.6 gigawatts through 2034. Only the first tranche was identified as purchased, so the headline capacity is not yet a firm order for the entire amount.
The reported Series G follows a $258 million Series F that Mainspring announced in April 2025. General Catalyst led that earlier round, which was intended to expand manufacturing and customer sales.
Mainspring said commercial shipments began in 2020 and that it now has more than one gigawatt in advanced development and field operations for Fortune 500 companies, data centers and utilities. That total mixes future projects with equipment already operating. The company describes the AEP agreement as a move beyond commercial, industrial and data-center markets into investor-owned utilities.