Grid batteries that last for days

How far are multi-day iron-air batteries from routine grid service?

Form Energy's iron-air batteries are being built into a utility pilot, a 1.5 MW system in Cambridge, Minnesota, rated for up to 100 hours of discharge. Great River Energy expected it to be running by late 2025. In October 2026, MIT Technology Review reported that it is set to come online in 2027. The sources reviewed publish no operating record, no round-trip efficiency and no installed cost. A 300 MW, 30 GWh project with Xcel Energy and Google is not due to ship its first modules until the end of 2028.

AI illustration · Illustrative scene, not a photograph of the specific device or experiment.
WHERE WE AREPilots

Close. Early pilots are running in the real world.

Pilot stage. A grid-connected 1.5 MW iron-air system rated for up to 100 hours was announced in August 2024 and described by Great River Energy as a first-of-its-kind multi-day storage project. The sources reviewed show that it is still being built and is not yet confirmed as operating, with start-up now reported for 2027. Form's West Virginia factory has begun producing batteries, but at a small scale compared with its contracted orders. No source reviewed shows a multi-day battery in routine utility service.

The tech tree

Everything recorded on Vrulum that this future needs, oldest foundations at the top and the goal at the bottom. Lines show what each step requires.

What it takes: Grid batteries that last for days Unlocked Being researched Locked
  1. 2023Iron-air batteriesResearch and prototypes Being researched.
  1. 2024US funding notice for non-lithium storage of 10 hours or moreDone Unlocked.
  2. 2024Construction of first grid-connected iron-air project announced in MinnesotaDone Unlocked.
  3. Date not recordedCambridge pilot running in commercial operationIn progress Being researched.
  4. Date not recordedFactory output at commercial scaleIn progress Being researched.
  5. Date not recordedPublished performance data from a grid-connected multi-day systemNot started Locked.
  6. Date not recordedLarge contracted projects deliveredNot started Locked.
  7. Date not recordedMulti-day storage valued in utility plans and tariffsNot started Locked.
  8. Date not recordedRoutine multi-day service across power gridsNot started Locked.
  1. GoalGrid batteries that last for daysGoalRequires: Iron-air batteries, US funding notice for non-lithium storage of 10 hours or more, Construction of first grid-connected iron-air project announced in Minnesota, Cambridge pilot running in commercial operation, Factory output at commercial scale, Published performance data from a grid-connected multi-day system, Large contracted projects delivered, Multi-day storage valued in utility plans and tariffs, Routine multi-day service across power grids. Locked.

The road so far

2 of 8 steps done, 2 in progress.

  1. Done · 2 Jul 2024US funding notice for non-lithium storage of 10 hours or more

    The US Department of Energy's Office of Clean Energy Demonstrations announced up to $100 million for non-lithium pilot projects with a discharge of at least 10 hours. It planned 3 to 15 projects of $5 million to $20 million each. [2]

  2. Done · 15 Aug 2024Construction of first grid-connected iron-air project announced in Minnesota

    Great River Energy and Form Energy announced the start of construction on a 1.5 MW grid-connected system in Cambridge, Minnesota. It is rated to deliver its rated power for up to 100 hours. The batteries were to be made at Form Factory 1 in Weirton, West Virginia. [1]

  3. In progressCambridge pilot running in commercial operation

    Great River Energy expected the project to be operational by late 2025. MIT Technology Review reported in October 2026 that a 150 MWh system for Great River Energy is set to come online in 2027. That article does not name Cambridge, so linking it to this pilot is an inference. The sources reviewed publish no operating data from the site. [1][3]

  4. In progressFactory output at commercial scale

    MIT Technology Review reports that Form has begun producing batteries at its West Virginia factory. The article gives no start date. It gives Form's existing production capacity as about 2 GWh of storage a year, against the 80 GWh it says it has promised through commercial agreements. [3]

  5. Not startedPublished performance data from a grid-connected multi-day system

    The sources reviewed contain no published operating record, availability figures or round-trip efficiency for a grid-connected iron-air system. A full multi-day dispatch event on a grid has not been reported.

  6. Not startedLarge contracted projects delivered

    Xcel Energy and Google plan a 300 MW, 30 GWh iron-air system to serve a new Google data centre in Minnesota. Form expects to ship its first modules for that project by the end of 2028, and MIT Technology Review reports that it is set to come online in phases between 2028 and 2031. [4][3]

  7. Not startedMulti-day storage valued in utility plans and tariffs

    Xcel and Google planned to file a tariff with Minnesota regulators under which Google would pay the build-out costs. The sources reviewed do not show a regulatory decision. [4]

  8. Not startedRoutine multi-day service across power grids

    No source reviewed shows iron-air or other multi-day batteries used routinely across power grids. Reaching this step would need several projects with published costs, efficiency and operating records.

Why it matters

Wind and solar output can stay low for several days at a time. A battery that discharges for days, rather than hours, could cover those gaps without running fossil backup plants. Whether it does so at a price utilities will pay depends on its efficiency, cost and how rarely it is used.

What stands in the way

  • Round-trip efficiency has not been published in the sources reviewed, so the energy lost in each charge and discharge cycle cannot be compared with other storage.
  • Installed cost has not been published. Form has said its goal is $20 per kWh, but it declined to share the current cost of its systems, according to MIT Technology Review.
  • The 1.5 MW Cambridge pilot has moved from a late 2025 start to a reported 2027 start, and no operating record exists in the sources reviewed.
  • Existing production capacity of about 2 GWh a year is far below the 80 GWh of commercial agreements Form cites.
  • Batteries that discharge over days may sit idle for long periods, so their economics depend on who pays for the capacity. MIT Technology Review describes this economics as tricky.
  • The tariff for the 300 MW Xcel and Google project was to be filed with Minnesota regulators, and the sources reviewed do not show a decision. Xcel has not yet decided where the battery will be sited.

Published forecasts

What others have predicted. These are their estimates, not ours.

WhoForecastTimeframeMade
Great River EnergyThe Cambridge, Minnesota project would be operational by late 2025. The sources reviewed show that this date has since passed without a reported start of commercial operation. [1]202515 Aug 2024
Form EnergyForm expects to ship its first modules for the 300 MW, 30 GWh Xcel and Google project by the end of 2028, as reported by Utility Dive in March 2026. [4]20284 Mar 2026

Signals to watch

  • Operating data from the Cambridge, Minnesota pilot, including availability, downtime and efficiency, published by Great River Energy or Form Energy.
  • A published round-trip efficiency figure stated on an AC-to-AC basis, with its test conditions.
  • Output from Form's West Virginia factory, and the date its first modules ship for the Xcel and Google project.
  • A decision by Minnesota regulators on the Xcel and Google tariff.
  • An installed cost per kWh from an operating project, set against Form's $20 per kWh goal.
  • New utility procurements for storage rated at 36 hours or more, with stated durations and prices.

Sources

  1. company announcement
    Great River Energy, Form Energy break ground on first-of-its-kind multi-day energy storage project
    Great River Energy, 15 Aug 2024.
  2. government report
    OCED Issues Notice of Intent for up to $100 Million for Non-Lithium Long-Duration Energy Storage Pilot Projects
    US Department of Energy, Office of Clean Energy Demonstrations, 2 Jul 2024.
  3. news
    2026 Climate Tech Companies to Watch: Form Energy and its iron batteries
    Jonathan W. Rosen. MIT Technology Review, 6 Oct 2026.
  4. news
    World's 'largest' grid battery part of Google-Xcel Energy agreement
    Utility Dive, 4 Mar 2026.

Last reviewed 21 Oct 2026.