Battery Runtime Estimator
Estimate battery runtime from nominal energy, a power load and an efficiency allowance.
How to use this calculator
- Enter amp-hour capacity, nominal voltage, load wattage and efficiency percentage.
- Confirm that all inputs use the stated units and the same population, period or scope.
- Change one assumption at a time when comparing scenarios.
Formula used by the battery runtime estimator calculator
Usable Wh = Ah × V × efficiency; hours = usable Wh ÷ W
- Usable Wh = Ah × V × efficiency; hours = usable Wh ÷ W
- Actual runtime varies with battery chemistry, age, temperature, discharge rate and inverter behaviour.
- Inputs are validated against required ranges and outputs are rounded for readability.
Worked examples
Worked example
100 Ah at 12 V, 120 W load and 85% usable efficiency.
Estimated runtime is about 8.5 hours.
Estimated runtime is about 8.5 hours.
Understanding the result
Estimate battery runtime from nominal energy, a power load and an efficiency allowance.
Actual runtime varies with battery chemistry, age, temperature, discharge rate and inverter behaviour.
Important notes
Actual runtime varies with battery chemistry, age, temperature, discharge rate and inverter behaviour.
Use the result as a calculation aid for the inputs supplied; confirm critical specifications independently.
Frequently asked questions
Why use voltage with Ah?
Multiplying amp-hours by nominal volts gives an approximate energy capacity in watt-hours.
Will a battery always deliver rated capacity?
No; rated capacity depends on discharge conditions and battery specifications.
How do I use this calculator?
Enter amp-hour capacity, nominal voltage, load wattage and efficiency percentage.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.