# Metal-air batteries: Engineering and performance

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

Source page: https://www.materialinsights.org/energy/e_metal_air/sg-eng/

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.^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)^[13](https://www.sandia.gov/app/uploads/sites/273/2021/12/3_Passell_Howard_SNL_NECPUC_Session1_3-19-2021.pdf)
 Form iron-air 35-38% (company, 2021-2023); about 40% (commentator)^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Lithium-ion benchmark About 85-90%^[13](https://www.sandia.gov/app/uploads/sites/273/2021/12/3_Passell_Howard_SNL_NECPUC_Session1_3-19-2021.pdf) Charging energy per MWh delivered at 38% About 2.6 MWh (calculated)^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)

#### 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).^[1](https://www.technologyreview.com/2026/10/06/1145020/2026-climate-tech-companies-to-watch-form-energy-iron-batteries/amp/)^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)^[3](https://www.energy-storage.news/form-energy-signs-definitive-agreement-for-100-hour-iron-air-bess-with-georgia-power/)
 Duration 100 h^[1](https://www.technologyreview.com/2026/10/06/1145020/2026-climate-tech-companies-to-watch-form-energy-iron-batteries/amp/)^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Georgia Power project 15 MW / 1,500 MWh^[3](https://www.energy-storage.news/form-energy-signs-definitive-agreement-for-100-hour-iron-air-bess-with-georgia-power/) Cost target USD 15-20/kWh^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Implied cost per kW at 100 h USD 1,500-2,000/kW (calculated)^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)

#### 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.^[1](https://www.technologyreview.com/2026/10/06/1145020/2026-climate-tech-companies-to-watch-form-energy-iron-batteries/amp/)^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)
 Land use About 1 acre per 2-3 MW^[1](https://www.technologyreview.com/2026/10/06/1145020/2026-climate-tech-companies-to-watch-form-energy-iron-batteries/amp/) Operating range -40 C to 50 C^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)

#### 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.^[5](https://www.eng.uwo.ca/nanoenergy/publications/2017/PDFs/a-comprehensive-review-on-recent-progress-in-aluminum-air-batteries.pdf)^[11](https://www.sgs.com/en/news/2023/09/safeguards-11523-eu-issues-new-regulation-on-batteries-and-waste-batteries)
 EU lead limit for zinc-air button cells 0.01% by weight from 18 Aug 2028^[11](https://www.sgs.com/en/news/2023/09/safeguards-11523-eu-issues-new-regulation-on-batteries-and-waste-batteries)

#### Sources

1. [15 Climate Tech Companies to Watch 2026: Form Energy (6 Oct 2026)](https://www.technologyreview.com/2026/10/06/1145020/2026-climate-tech-companies-to-watch-form-energy-iron-batteries/amp/) 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)](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Energy-Storage.news, 2025
3. [Form Energy signs definitive agreement for 100-hour iron-air BESS with Georgia Power](https://www.energy-storage.news/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)](https://www.eng.uwo.ca/nanoenergy/publications/2017/PDFs/a-comprehensive-review-on-recent-progress-in-aluminum-air-batteries.pdf) KeAi / Western University, 2017
5. [SafeGuardS 115/23: EU issues new regulation on batteries and waste batteries (summary of Regulation (EU) 2023/1542)](https://www.sgs.com/en/news/2023/09/safeguards-11523-eu-issues-new-regulation-on-batteries-and-waste-batteries) SGS, 2023
6. [Energy Storage Overview (H. Passell, NECPUC Energy Storage Webinar Series, 19 Mar 2021)](https://www.sandia.gov/app/uploads/sites/273/2021/12/3_Passell_Howard_SNL_NECPUC_Session1_3-19-2021.pdf) Sandia National Laboratories, 2021

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_metal_air/index.md)
- [Overview](https://www.materialinsights.org/energy/e_metal_air/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_metal_air/correlation/index.md)
- [Science and technology](https://www.materialinsights.org/energy/e_metal_air/sg-sci/index.md)
- [Economics](https://www.materialinsights.org/energy/e_metal_air/sg-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_metal_air/sg-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_metal_air/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_metal_air/links/index.md)