Battery Runtime Calculator
The Battery Runtime Calculator estimates how long an electrical load can run from a battery when usable energy and power draw are known. Watt-hours are the energy capacity unit, while watts describe the rate of energy use.
How to use this calculator
- Enter battery energy capacity in watt-hours, device power draw in watts, and a realistic usable-energy efficiency percentage.
- Review the worked device runtime and compare its arithmetic with the inputs you plan to use.
- Change one input at a time when exploring a different scenario, and keep units and assumptions consistent.
Estimated operating hours
Runtime in hours = battery capacity in Wh × usable-energy fraction ÷ load in W.
- The usable-energy factor accounts for an allowance that real systems may lose before the battery reaches its rated endpoint.
- The page's numerical output is calculated from the values entered; it does not independently verify whether those inputs fit the real-world situation.
Worked examples
Device runtime
A 500 Wh battery supplies a 100 W load with 85% usable energy.
500 × 0.85 ÷ 100 = 4.25 hours
Estimated runtime is 4.25 hours.
Understanding the result
A higher load reduces runtime while increasing the usable energy increases it proportionally.
Battery age, temperature, peak loads, inverter efficiency, voltage cut-off, and the manufacturer's ratings affect real performance.
Important notes
Use the system's actual watt-hour rating and do not infer Wh from amp-hours without a voltage value.
Displayed values may be rounded for readability. Keep appropriate precision for intermediate calculations and check the unit labels before using the result.
Frequently asked questions
What does the Battery Runtime Calculator result mean?
A higher load reduces runtime while increasing the usable energy increases it proportionally.
What should I check before using the battery runtime calculator?
Use the system's actual watt-hour rating and do not infer Wh from amp-hours without a voltage value.
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Learn the method behind the number or compare the terminology before using the calculator.
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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.