Battery Life from Load Calculator
How long a battery lasts depends on how much current the load draws against the battery's usable capacity. Enter the battery capacity in amp-hours, the load current in amps, and a usable-capacity derating factor. The calculator estimates the runtime in hours and minutes. The derating factor allows for the gap between rated and real-world capacity caused by discharge rate, temperature, and age. Treat the result as a planning estimate, not an exact figure.
Battery runtime formula
Runtime (hours) = (capacity_Ah * derating factor) / load_A
where derating factor is between 0 and 1
If your load is in watts, first convert to amps by dividing watts by the battery voltage. The derating factor reflects the usable share of nameplate capacity.
Worked example
A 100 Ah battery supplies a 5 A load with a usable factor of 0.8. Runtime = (100 * 0.8) / 5 = 80 / 5 = 16.00 hours. That is 16 hours and 0 minutes of estimated runtime.
Battery life from load: frequently asked questions
How is battery runtime estimated?
Runtime in hours is the usable capacity divided by the load current: hours = (capacity in Ah times derating factor) / load in amps. The derating factor accounts for the fact that you rarely get the full nameplate capacity in practice.
Why use a derating factor?
Real batteries deliver less than their rated amp-hours because of internal resistance, temperature, age, and the Peukert effect at higher discharge rates. A factor of 0.8 (80 percent usable) is a common conservative default; lead-acid users often go lower, lithium higher.
What units should I enter?
Capacity in amp-hours (Ah) and load current in amps (A). If your load is given in watts, divide it by the battery voltage to get amps first. Runtime is returned in hours.
Is this an exact figure?
No. It is an estimate. Actual runtime depends on discharge rate, temperature, battery chemistry, and age. Treat the result as a planning figure and keep a safety margin.
Sources
- NIST: SI units (ampere, hour).
- The capacity-over-current runtime relation is a direct definition of the amp-hour. The derating factor is a user-editable estimate.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.