Material Insights · Understanding the Material World

Lead-acid batteries: Engineering and performance

Why only half the rated energy is really usable

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

Round-trip efficiency, 2 h / 6 h / 10 h 77% / about 82% / 85%2 Literature range 75-84%2

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

Typical depth of discharge 50-80%2 Energy available at a 1-hour discharge rate about 50% of rated2

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

Volumetric energy density 70-135 kWh/m31 Lithium-ion for comparison 200-700 kWh/m31

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.26

NFPA 855 threshold, VRLA vs lithium-ion about 70 kWh vs 20 kWh6 Project markup, lead-acid vs lithium-ion (PNNL) 15% vs 20%2

Sources

  1. Sodium-ion batteries: Technology brief (includes lead-acid and lithium-ion comparison data) IRENA, 2025
  2. Lead-Acid Battery methodology, 2020 Grid Energy Storage Technology Cost and Performance Assessment PNNL / US DOE, 2020
  3. NFPA 855: 2026 edition updates and what they mean for energy storage projects Energy-Storage.news, 2026