The Battery That Runs on Rust Just Landed Its Biggest Deal Yet

Lithium-ion batteries solved the easy part of grid storage: smoothing out a few hours of solar or wind variability. They're a much worse fit for the harder problem — a multi-day stretch of cloudy, windless weather, or a genuine grid emergency — because storing that much energy in lithium cells for that long is prohibitively expensive. Form Energy's iron-air battery is built specifically for that gap, and in 2026 it picked up a deployment significant enough to suggest the idea is more than a lab curiosity: a 300-megawatt, 30-gigawatt-hour system backing a new Google data center in Minnesota.

How a battery runs on rust

Form Energy's technology works on what's essentially reversible rusting. Each battery module, roughly the size of a washing machine, uses iron, water, and air as its active materials. During discharge, the iron electrode pulls oxygen from the air and oxidizes — literally turns to rust — releasing electricity as it does. Charging reverses the reaction, converting the rust back to metallic iron and releasing oxygen. The electrolyte is water-based and non-flammable, and the raw materials (iron, water, air) are about as cheap and abundant as inputs get.

What "100 hours" actually buys

The headline spec is that Form's battery can discharge continuously for up to 100 hours at rated power — roughly four days — compared to the 2-to-4-hour discharge window typical of a lithium-ion grid battery. That's not a marginal improvement, it's a different use case entirely: lithium batteries smooth daily solar and wind cycles, while a 100-hour iron-air system is aimed at multi-day weather events, seasonal shortfalls, or genuine grid resilience scenarios that shorter-duration storage simply can't cover.

That's not a marginal improvement, it's a different use case entirely.

The utility deals that came first

Before the Google-backed project, Form Energy's clearest commercial validation came from Xcel Energy, an eight-state utility that contracted for two smaller 10-megawatt, 100-hour systems — each storing a gigawatt-hour of energy — at sites in Pueblo, Colorado, and Becker, Minnesota. Those earlier contracts were the first evidence that a regulated utility was willing to commit real money to multi-day storage rather than just running pilot projects.

Why a hyperscaler is paying for this

The new Minnesota project is a different order of magnitude: Google is directly funding a 300-megawatt, 30-gigawatt-hour iron-air system as part of a broader deal with Xcel Energy that also adds 1.4 gigawatts of wind and 200 megawatts of solar to serve a new data center. Google is picking up the (still undisclosed) bill and has committed to covering any new grid-infrastructure costs so other Xcel customers aren't left subsidizing one company's data center — the same pattern of hyperscalers directly bankrolling firm power that's also showing up in geothermal deals covered elsewhere on this site. Battery deliveries are expected to begin in 2028, with the surrounding clean energy build-out phased in through 2031.

The honest caveats

Iron-air batteries are heavier and less energy-dense than lithium-ion, which is exactly why they're not competing to replace lithium in short-duration applications, or in anything that needs to be compact or mobile — they're a complement, not a replacement. And a technology's economics at hyperscaler contract scale still depend on Form Energy successfully manufacturing and deploying at volume by its 2028 delivery target, which is a different challenge than building a handful of smaller pilot systems, however promising the chemistry looks on paper.