Battery calculator
Lead-acid batteries deliver less than their rated amp-hours when discharged faster than the test rate. Peukert's law estimates that reduction from the battery's rated current and exponent.
Calculation breakdown
| Rated current100 Ah ÷ 20 h | 5 A |
|---|---|
| Effective capacity | 70.71 Ah |
| Capacity reduction | 29.3% |
Effective Ah = Rated Ah × (rated current ÷ actual current)^(n − 1) = 100 × (5 ÷ 20)^(1.25 − 1) = 70.71 Ah; runtime = 3 h 32 min
Method
A 100 Ah C20 battery is rated at 5 A. Drawing substantially more than 5 A reduces effective capacity. The exponent describes how strongly capacity shrinks as current rises.
The model is a rough approximation and is most relevant to lead-acid. Lithium batteries usually use an exponent close to 1 and should be modeled from their discharge curves and BMS cutoff instead.
I_rated = Capacity ÷ rated hours C_effective = Capacity × (I_rated ÷ I_actual)^(n − 1) Runtime = C_effective ÷ I_actual
Symbols
Worked examples
Example
A lead-acid battery rated 100 Ah over 20 hours is discharged at 20 A with exponent 1.25.
Rated current = 100 Ah ÷ 20 h = 5 A Effective capacity = 100 × (5 ÷ 20)^0.25 = 70.71 Ah Runtime = 70.71 Ah ÷ 20 A = 3.54 h
Result: About 3 h 32 min.
Limits of the model
Assumptions
Current is constant and the exponent matches the battery and temperature.
Label capacity is measured at the entered rated discharge time.
Not covered
Peukert is only an approximation and can understate losses at very high current.
It does not model voltage cutoff, temperature, age or intermittent loads.
Sources
Next steps
C-rate compares current with battery capacity. A 100 Ah battery at 20 A is running at 0.2 C; the same 20 A is 2 C for a 10 Ah battery.
Enter your battery and your load to estimate runtime. The calculator applies depth of discharge, battery health, conversion losses and — for lead-acid only — the Peukert effect, and shows each step of the energy budget.
The label capacity is not the energy available to your load. Set the starting and minimum state of charge, battery health and conversion efficiency to expose the usable window.
Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.