Fourth Power Thermal Battery

Fourth Power is an energy storage company at 44 Middlesex Turnpike, Bedford, Massachusetts. Its company page says it launched in 2023 from a decade of federally funded research at MIT and Georgia Tech, and aims to deliver peak power at or below the lifetime cost of new gas.

The thermal battery converts low-cost electricity to heat held in graphite blocks in an insulated inert environment, and returns it as electricity on demand. The listed baseline is 25 MW and 50 hours on a single acre, scalable to 100+ hours, with no water use. Backers include DCVC, Breakthrough Energy, MassMutual Catalyst and Munich Re Ventures.

• Graphite thermal storage: electricity is stored as heat in insulated graphite blocks.
• Multi-day duration: 50-hour baseline, scalable to 100+ hours.
• Grid-scale output: 25 MW per acre as listed.
• Water-free operation: no water consumption.
• Demonstration: first full-scale demo began commissioning in September 2026.

Peaking capacity for utilities replacing gas peakers. Multi-day firming for large solar parks. Long-duration backup for industrial zones with constrained grids.

Multi-day storage of 50 hours or more from a single-acre footprint.

No water consumption and 0 percent thermal runaway risk, per the site.

Lifetime cost stated to be below gas peakers and lithium-ion.

Compact design suits sites with limited land.

Grids with high solar and wind shares need multi-day storage to cover low-renewable periods. Thermal batteries aim to compete with gas peakers on cost. Vietnam and ASEAN markets are expanding solar and are studying how to replace gas and coal peaking capacity.

Bedford, Massachusetts. Launched 2023 per the company page. A US$20 million round led by Munich Re Ventures with DCVC and Breakthrough Energy Ventures was announced in September 2025. Offerings are at pre-commercial demonstration stage.