Sizing & runtime

Battery Runtime Calculator

Estimate how long a battery will power a load, with optional Peukert correction for lead-acid batteries.

Rated capacity

Battery voltage

Load

Usable depth of discharge

Conversion efficiency (power loads)

Peukert exponent

1.00 = ideal (≈ lithium). Lead-acid is typically 1.1–1.3; see the datasheet.

Rated discharge time (C-rate hours)

Formula

I = P ÷ (V × η)   (or the current entered)
t(full) = H × (C ÷ (I × H))^k
t(usable) = t(full) × DoD

100 Ah @ C20, 12 V, 60 W, k = 1, DoD 50% → I = 5 A → 20 h full → 10 h usable.

Peukert's law is an approximation that works best near constant current and room temperature.
Real loads cycle on and off; use the average current.

Nominal capacity (Ah): Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
Usable capacity: The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
Cranking current (CCA): Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
Stored energy (Wh): Ah × voltage. The right unit to compare batteries of different voltages.