# Lead-acid batteries: Engineering and performance Why only half the rated energy is really usable Source page: https://www.materialinsights.org/energy/e_lead_acid/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. Lead-acid loses more energy per cycle than lithium-ion, can only use part of its rated capacity, and takes up a lot of space, but it does not suffer lithium-style thermal runaway. #### Round-trip efficiency Mixed PNNL found about 82% round-trip efficiency for a 6-hour system, 77% at 2 hours and 85% at 10 hours, because lead-acid loses more at high discharge rates. Literature values run from 75 to 84%. That is several points below lithium-ion's 85 to 95%.^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) Round-trip efficiency, 2 h / 6 h / 10 h 77% / about 82% / 85%^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) Literature range 75-84%^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) #### Power and energy decoupling Unfavourable Power and energy are tied to the number of cells. Lead-acid capacity is rated at a very slow discharge (over 50 to 100 hours); at a 1-hour rate only about half of that energy is available, and to protect life the battery is normally cycled between 50% and 80% depth of discharge. A buyer therefore installs roughly 1.25 to 2 times the energy that will be used each day (calculated from the 50 to 80% depth range), more for fast discharges. This makes extra hours of storage expensive.^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) Typical depth of discharge 50-80%^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) Energy available at a 1-hour discharge rate about 50% of rated^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) #### Footprint and siting flexibility Unfavourable At 70 to 135 kWh per cubic metre, lead-acid needs several times the volume of lithium-ion for the same energy, and its weight makes rooftop or upper-floor siting hard. Flooded batteries release hydrogen while charging, so battery rooms need ventilation. For homes and small offices, this is manageable; for grid-scale sites it is a serious handicap.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2025/Nov/IRENA_TEC_Sodium-ion_batteries_2025.pdf) Volumetric energy density 70-135 kWh/m3^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2025/Nov/IRENA_TEC_Sodium-ion_batteries_2025.pdf) Lithium-ion for comparison 200-700 kWh/m3^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2025/Nov/IRENA_TEC_Sodium-ion_batteries_2025.pdf) #### Safety and thermal runaway risk Favourable Lead-acid uses a water-based electrolyte that does not burn, so it avoids the fire propagation risks of lithium-ion. The main hazards are hydrogen build-up, acid spills and lead exposure. NFPA 855 applies to VRLA only above about 70 kWh, against 20 kWh for lithium-ion, and lead-acid may be exempt from hazard mitigation analysis in some cases. PNNL applied a lower project markup for lead-acid (15% versus 20%) partly because of fewer safety-related costs.^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf)^[6](https://www.energy-storage.news/nfpa-855-2026-edition-updates-and-what-they-mean-for-energy-storage-projects/) NFPA 855 threshold, VRLA vs lithium-ion about 70 kWh vs 20 kWh^[6](https://www.energy-storage.news/nfpa-855-2026-edition-updates-and-what-they-mean-for-energy-storage-projects/) Project markup, lead-acid vs lithium-ion (PNNL) 15% vs 20%^[2](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) #### Sources 1. [Sodium-ion batteries: Technology brief (includes lead-acid and lithium-ion comparison data)](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2025/Nov/IRENA_TEC_Sodium-ion_batteries_2025.pdf) IRENA, 2025 2. [Lead-Acid Battery methodology, 2020 Grid Energy Storage Technology Cost and Performance Assessment](https://www.pnnl.gov/sites/default/files/media/file/LeadAcid_Methodology.pdf) PNNL / US DOE, 2020 3. [NFPA 855: 2026 edition updates and what they mean for energy storage projects](https://www.energy-storage.news/nfpa-855-2026-edition-updates-and-what-they-mean-for-energy-storage-projects/) Energy-Storage.news, 2026 ## Other sections - [Summary](https://www.materialinsights.org/energy/e_lead_acid/index.md) - [Overview](https://www.materialinsights.org/energy/e_lead_acid/overview/index.md) - [Correlation](https://www.materialinsights.org/energy/e_lead_acid/correlation/index.md) - [Science and technology](https://www.materialinsights.org/energy/e_lead_acid/sg-sci/index.md) - [Economics](https://www.materialinsights.org/energy/e_lead_acid/sg-econ/index.md) - [Impact and additionality](https://www.materialinsights.org/energy/e_lead_acid/sg-impact/index.md) - [What to track](https://www.materialinsights.org/energy/e_lead_acid/tracking/index.md) - [Relationships](https://www.materialinsights.org/energy/e_lead_acid/links/index.md)