Flatiron Energy secures $850M for 1.2GWh Energizar BESS near Boston

Key takeaways

• Flatiron Energy closed about US$850 million to build a 1,200MWh battery project near Boston.
• Due online in Q4 2027, it would be the largest battery site on the US east coast.
• Banks, a big investment fund and two Fortune 500 firms are funding it.
• Open question: the plant's exact power rating, listed at 250MW but possibly 300MW.
• Next: Massachusetts is shopping for at least 1,000MW more storage.

What happened

Flatiron Energy, a US storage developer, has closed roughly US$850 million in funding. The money will build Energizar, a battery project in Chelsea, Massachusetts. Flatiron announced the deal on 15 October.

Energizar is a BESS, short for battery energy storage system: a big bank of batteries that charges from the grid and sends power back when it is needed. It will hold 1,200MWh. A megawatt-hour (MWh) is a measure of stored energy, roughly what a 1MW plant delivers in one hour.

Who is paying

The deal mixes several kinds of money. A group of banks led by Nord L/B, Santander and Societe Generale is lending for construction. Investec and East West Bank joined as lead arrangers, and Siemens Financial Services is also a lender.

Funds run by an unnamed credit manager with more than US$350 billion under management are putting in preferred equity, an ownership stake that gets paid before ordinary shareholders. Two Fortune 500 companies will buy part of the project's tax credits. These US credits reward clean energy investment and can be sold to other firms.

Why it matters

Flatiron says Energizar will be the largest battery site on the US east coast once it starts in the fourth quarter of 2027. It will supply backup capacity and power at peak times for communities on the Boston grid.

The project came out of a state tender. In late 2025, the Massachusetts Department of Energy Resources (DOER), the state energy agency, picked 1,268MW of storage projects. Energizar was one. Flatiron's 168MW Salt Cod project, at a former coal plant site, was another.

The open question on power

Stored energy and power are two different numbers. Energy (MWh) is how much the battery holds. Power (MW) is how fast it can deliver it. Flatiron still lists Energizar at 250MW, a figure from when the project was planned at 1,000MWh. Energy-Storage.news says it could be about 300MW if the four-hour design stays, but that is unconfirmed.

What's next

Massachusetts wants more storage. DOER has asked for bids on 1,000MW of mid-duration batteries, plus 250MW to 300MW of smaller projects on local power lines. The state has also set out principles for how data centers should be developed there.