What the business is
Cambridge, Massachusetts startup developing long-duration thermal batteries for the grid
The bet
Storing grid power as heat in carbon blocks and converting it back with thermophotovoltaics can undercut lithium-ion and gas peaker plants on cost
Starting capital
$20 million Series A Plus after a $19 million Series A in 2023
How it started
Fourth Power, a four-year-old Cambridge, Massachusetts startup, stores electricity by heating blocks of carbon in argon-filled insulated chambers. To release power, it pumps molten tin heated to about 2,400°C through graphite pipes, and thermophotovoltaic cells convert the tin's infrared light back into electricity. Insulation made from petroleum coke loses only 1% of stored energy per day.
What happened
The company spent two years refining the technology and was running cycle tests on smaller systems to check power output and durability. It raised a $19 million Series A in 2023, and in 2025 a $20 million Series A Plus, led by Munich Re Ventures with Breakthrough Energy Ventures and DCVC, to design a 1-megawatt-hour demonstration battery.
How it ended up
As of September 2025 Fourth Power planned to build its first full-scale battery and deliver commercial-scale batteries to customers in 2028. CEO Arvin Ganesan said first-of-a-kind units should be cost competitive and that, at scale, costs could fall to $25 per kWh, one-tenth of lithium-ion; these are projections, not results.
What has to be true
Long-duration storage that is cheap enough would make solar and wind viable around the clock
Carbon blocks, tin and petroleum-coke insulation are cheap materials with few supply-chain players
Eight-plus hour discharge is twice what most grid lithium-ion systems are designed for
After two years of refinement, the Series A Plus funds a 1 MWh demonstration unit
What can be applied
Long-duration storage competes on cost per kWh and hours, so a startup picks cheap materials and few suppliers, but its figures remain projections until a full-scale unit runs.
Aftermath
As of 2025-09-16, Fourth Power was testing smaller systems and designing a 1 MWh demonstration battery, with commercial-scale deliveries targeted for 2028 and a $25/kWh cost goal that was unproven.
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