Free energy calculator
Battery Runtime Calculator
Estimate how long a battery may support a steady electrical load. The calculator keeps practical losses visible instead of promising an exact runtime.
Calculate your estimate
Change the example values to match your system.
Estimated result
Ideal—
Typical—
Conservative—
These are estimates, not guaranteed performance.
Formula and method
Nominal Wh = volts × amp-hours. Typical hours = nominal Wh × depth of discharge × efficiency ÷ (load watts + standby watts).
Percent fields are entered as percentages; the calculation converts them to decimal fractions. Displayed results are rounded to two decimal places.
What the inputs mean
- Battery voltage (V)
- Enter battery voltage in V.
- Nominal capacity (Ah)
- Enter nominal capacity in Ah.
- Load (W)
- Enter load in W.
- Depth of discharge (%)
- Enter depth of discharge in %.
- Conversion efficiency (%)
- Enter conversion efficiency in %.
- Standby consumption (W)
- Enter standby consumption in W.
- Conservative retained fraction (%)
- Enter conservative retained fraction in %.
Worked example
A 12 V, 100 Ah battery at 80% DoD and 90% efficiency supplying 100 W plus 5 W standby gives about 8.23 typical hours.
Why the real result may differ
- Temperature, battery age, wiring resistance, and changing loads can move actual results away from the estimate.
- Manufacturer ratings may be measured under different test conditions. Measure the load when practical.
Common mistakes
- Do not confuse watts (power) with watt-hours (energy).
- Do not enter a percentage as a whole number: 90% is entered as 90 in this form and converted internally.
Frequently asked questions
Is this result guaranteed?
No. It is an engineering estimate; equipment, temperature, age, and duty cycle affect real performance.
Why are three estimates shown?
The ideal case ignores practical losses, while typical and conservative cases apply the efficiency and margin entered above.
Related tools
References
Last reviewed: August 6, 2026