Google announced earlier this week that it was building a new data center in Minnesota that would be powered by a mix of wind, solar, and a very unique battery built by startup Form Energy that’s capable of discharging for days on end.

Now we know the price tag for that feat of electrochemical engineering: about $1 billion, according to The Information.

Form Energy’s massive iron-air battery is capable of delivering a continuous 300 megawatts of electricity over 100 hours. It works by breathing, in a sense — oxygen pumped into the cells rusts iron, which releases electrons. The battery will work to smooth the flow of electrons from 1.4 gigawatts of wind power and 200 megawatts of solar power.

The startup has been chipping away at the technology for years, and it has built a factory in West Virginia to produce the batteries. But it hadn’t landed a major customer until this recent deal with Google.

With a big order on the books, Form Energy CEO Mateo Jaramillo said that his company is in the process of raising a $500 million round. Form has raised $1.4 billion to date, according to PitchBook. The company plans to go public next year.