Free energy calculator
Battery Capacity Calculator
Work backward from a load and desired runtime to estimate the nominal battery capacity you need.
Calculate your estimate
Change the example values to match your system.
Estimated result
Ideal—
Typical—
Conservative—
Enter valid inputs and calculate to see results.
These are estimates, not guaranteed performance.
Formula and method
Required Ah = (load + standby) × hours ÷ (voltage × DoD × efficiency).
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
- Load (W)
- Enter load in W.
- Target runtime (hours)
- Enter target runtime in hours.
- Battery voltage (V)
- Enter battery voltage in V.
- Depth of discharge (%)
- Enter depth of discharge in %.
- Conversion efficiency (%)
- Enter conversion efficiency in %.
- Output-side standby load (W)
- Enter any additional output-side load not already included in the measured load.
- Conservative retained fraction (%)
- Enter conservative retained fraction in %.
Worked example
A 100 W load for 8 hours on a 12 V system at 80% DoD and 90% efficiency needs about 97.22 Ah before an extra margin.
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 12, 2026