Material Insights · Understanding the Material World

Metal-air batteries: Engineering and performance

Why throwing away 60% of the power can still make sense

What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.

Iron-air gives up efficiency for very low cost per kWh: about 35 to 40% round-trip efficiency against 85 to 90% for lithium-ion. That only makes sense for rare, multi-day discharges.

Round-trip efficiency

Unfavourable

Form Energy's own presentations from 2021 to 2023 gave 35 to 38% round-trip efficiency, and US commentator Peter Kelly-Detwiler cited about 40% on LinkedIn; neither is independently measured, and Ore Energy has published no figure (assumed at 40 to 50%). Lithium-ion reaches about 85 to 90%. Losses arise mainly at the air electrode, where oxygen reactions need a large voltage push in both directions. Low efficiency means each MWh delivered needs about 2.6 MWh of charging (calculated at 38%), so the system must charge from very cheap surplus power.213

Form iron-air 35-38% (company, 2021-2023); about 40% (commentator)2 Lithium-ion benchmark About 85-90%13 Charging energy per MWh delivered at 38% About 2.6 MWh (calculated)2

Power and energy decoupling

Mixed

Iron-air modules are designed for a fixed ratio of about 100 hours, so a 15 MW plant holds 1,500 MWh. Energy is cheap and power is relatively expensive, the reverse of lithium-ion. Form's goal of USD 15 to 20 per kWh (a future target, not a current price) would equal about USD 1,500 to 2,000 per kW of discharge capacity at 100 hours (calculated).123

Duration 100 h12 Georgia Power project 15 MW / 1,500 MWh3 Cost target USD 15-20/kWh2 Implied cost per kW at 100 h USD 1,500-2,000/kW (calculated)2

Footprint and siting flexibility

Mixed

Iron-air is bulky: Form's plants need about one acre per 2 to 3 MW, and the planned 30 GWh system for a Google data centre in Minnesota would cover at least 75 football fields' worth of land. Form gives an operating range of -40 to 50 C for its containerised units. Space is the main siting constraint, which favours rural sites near transmission lines or retiring power plants.12

Land use About 1 acre per 2-3 MW1 Operating range -40 C to 50 C2

Safety and thermal runaway risk

Favourable

Iron-air and zinc-air cells use water-based electrolytes that do not burn, so fire risk is low. The hazards are caustic electrolyte leaks and, for aluminium-air, hydrogen released by corrosion, which needs ventilation. US grid installations still go through NFPA 855 permitting, and in the EU portable zinc-air button cells must meet a 0.01% lead limit from 18 Aug 2028.511

EU lead limit for zinc-air button cells 0.01% by weight from 18 Aug 202811

Sources

  1. 15 Climate Tech Companies to Watch 2026: Form Energy (6 Oct 2026) MIT Technology Review, 2026
  2. 100-hour LDES battery technologies from Form, Noon and Ore: how do they compare? (cites Form presentations and a LinkedIn commentator) Energy-Storage.news, 2025
  3. Form Energy signs definitive agreement for 100-hour iron-air BESS with Georgia Power Energy-Storage.news, 2023
  4. A comprehensive review on recent progress in aluminum-air batteries (Liu et al., Green Energy and Environment) KeAi / Western University, 2017
  5. SafeGuardS 115/23: EU issues new regulation on batteries and waste batteries (summary of Regulation (EU) 2023/1542) SGS, 2023
  6. Energy Storage Overview (H. Passell, NECPUC Energy Storage Webinar Series, 19 Mar 2021) Sandia National Laboratories, 2021