Peukert Battery Capacity Calculator for Lead-Acid Batteries

Enter the capacity printed for the battery, the hour rate used for that rating, the expected load current, and the battery's Peukert exponent. The result includes the implied rated current, adjusted effective capacity, estimated runtime, and delivered capacity as a percentage of the nameplate rating. It is useful for comparing realistic backup or off-grid performance with a manufacturer's laboratory rating.

Start with the complete battery rating

A lead-acid capacity figure is incomplete unless its rating period is known. A battery marked 100 Ah at the 20-hour rate was characterized at 5 A, because 100 amp-hours divided by 20 hours equals 5 amperes. Enter that nameplate capacity as rated_capacity_ah and the associated period as rated_discharge_hours. Then enter the constant current your equipment is expected to draw. Use amperes, not watts; if the load is specified in watts, first estimate battery-side current while allowing for system voltage and inverter efficiency. Finally, enter a Peukert exponent supplied by the battery maker or obtained from suitable discharge tests. An exponent of 1 represents an ideal battery whose available amp-hours do not change with discharge rate. Real lead-acid batteries normally have an exponent above 1, so high currents reduce usable capacity. Matching the rating period to the quoted capacity is essential. Combining a 20-hour capacity with a 10-hour period silently creates the wrong reference current and therefore the wrong result.

Understand the calculation and its outputs

The calculator first derives the rated current by dividing rated capacity by rated discharge hours. It then applies Peukert's relationship, treating the manufacturer's rated current and time as the reference point. Estimated runtime equals the rated hours multiplied by the ratio of rated current to actual current, raised to the Peukert exponent. Effective capacity is that runtime multiplied by the actual current. The capacity percentage compares this adjusted capacity with the nameplate figure. At a current above the rated current, the estimate normally shows fewer usable amp-hours and a percentage below 100. At a lower current, the mathematical model can show more than the rated capacity because the battery is being discharged more gently than it was during its rating test. This does not create energy; it describes how the empirical lead-acid discharge relationship shifts usable charge with current. Results are rounded only for stable presentation. The underlying calculation uses the supplied decimal values directly and performs no lookup, interpolation, or random adjustment.

Use the estimate within practical limits

Peukert's equation is a planning model, not a complete battery simulator. It assumes a steady discharge current and a representative exponent. Loads that pulse, cycle, or change with inverter behavior should be converted into a defensible current profile, and a single-current estimate should be treated as an approximation. Temperature, battery age, state of charge, cable losses, cutoff voltage, manufacturing variation, and prior cycling can all reduce real runtime. The equation is most useful for comparing load choices on the same battery or checking whether a nameplate rating is being applied at a very different current. For conservative system design, include a reserve and respect the manufacturer's permitted depth of discharge; effective capacity is not the same as recommended usable capacity. Validate critical backup, medical, safety, or remote-power designs with current battery documentation and a controlled load test. The browser calculation is free, while automated API requests use the published $0.002 per request. No network service, battery database, or stored measurement is involved in the calculation.