Battery Amp-Hour Runtime Calculator
Battery runtime is the time a battery can supply a given load before its usable charge runs out. The simplest model divides the usable amp-hour capacity by the load current. This calculator takes the rated capacity, a usable depth-of-discharge fraction, and the load current, then returns the runtime in hours and minutes plus the usable capacity in amp-hours. The usable fraction is a user input because the safe depth of discharge depends on your battery chemistry. Use it to size batteries for off-grid, backup, and portable power systems.
Battery runtime formula
usable capacity = capacity (Ah) * usable fraction
runtime (hours) = usable capacity / load current (A)
runtime (minutes) = runtime (hours) * 60
depth of discharge (%) = usable fraction * 100
This is the linear amp-hour model. It assumes capacity does not depend on discharge rate. Real batteries deliver less at high loads, so treat the result as an optimistic upper bound.
Practical notes
- Lead-acid is often limited to about 50 percent depth of discharge for long life.
- Lithium chemistries usually tolerate 80 percent or more usable depth.
- Convert watt-hours to amp-hours by dividing by the pack voltage.
- Convert a load in watts to amps by dividing by the battery voltage.
- High discharge rates reduce usable capacity (the Peukert effect) beyond this model.
Battery runtime: frequently asked questions
How do you calculate battery runtime from amp-hours?
Runtime in hours is the usable capacity divided by the load current: runtime = (capacity in Ah * usable fraction) / load in amps. For example, a 100 Ah battery using 80 percent of its capacity at a 5 A load lasts (100 * 0.8) / 5 = 16 hours.
Why include a usable fraction or depth of discharge?
Few batteries should be fully drained. Lead-acid is often limited to about 50 percent depth of discharge for long life, while lithium chemistries tolerate more. The usable fraction is a user input because the right value depends on your battery chemistry and the lifespan you want.
Does runtime scale linearly with load?
This calculator assumes a linear amp-hour model: half the load gives double the runtime. Real batteries deliver less usable capacity at high discharge rates (the Peukert effect), so heavy loads run down faster than the simple formula predicts.
How do I convert watt-hours to amp-hours?
Divide watt-hours by the battery voltage: Ah = Wh / V. A 12 V battery rated 600 Wh holds 50 Ah. Enter the amp-hour figure here, or convert your watt-hour rating first using the nominal pack voltage.
What load current should I use?
Use the average current your device draws. If you know the power in watts, divide by the battery voltage to get amps. For mixed loads, estimate a weighted average. The runtime is only as accurate as the load figure you provide.
Official sources
- NIST: SI units (ampere, hour, coulomb).
- U.S. Department of Energy: Batteries and energy storage.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.