Fourth Power is a four-year-old startup based in Cambridge, Massachusetts that is building grid-scale thermal batteries designed for eight-plus hours of discharge. The system stores electricity by heating carbon blocks in argon-filled, insulated chambers and later pumps 2,400 °C molten tin through graphite pipes; thermophotovoltaic cells then convert the emitted infrared heat back into electricity. Thanks to an insulation layer made from petroleum coke, the battery loses only about 1 % of its stored energy per day. The company is currently testing smaller prototypes and is designing a 1 MWh demonstration unit. Management projects first commercial systems in 2028 at costs that undercut both lithium-ion batteries and peaking natural-gas plants, ultimately targeting $25 per kilowatt-hour—roughly one-tenth today’s lithium-ion costs. Fourth Power has raised $39 million to date, including a recently closed $20 million Series A Plus round and a $19 million Series A in 2023, to fund technology development and scale-up.

Total raised

$39M

Funding rounds

2

Latest round

Series A

Latest activity

Sep 2025