battery tool
Battery Capacity Calculator
Estimate nominal battery capacity needed for a target runtime.
This estimates nominal Ah capacity needed for the target load and runtime after DoD and efficiency assumptions.
Required nominal capacity
46.3 Ah
Capacity = load × hours ÷ (volts × DoD × efficiency).
Battery capacity guide
Estimate the battery capacity needed for a load
Use the connected load and desired runtime to estimate nominal battery capacity. The calculator accounts for system voltage, selected DoD, and inverter efficiency.
Formula and variables
Ah = load W × runtime hours ÷ (V × DoD × efficiency)
Load is in watts, runtime is in hours, voltage is in volts, and DoD and efficiency are fractions between 0 and 1 internally.
Worked examples
Small backup: 100 W for 8 hours at 12 V, 80% DoD, and 90% efficiency requires about 74.07 Ah.
Larger system: 1,000 W for 4 hours at 48 V, 80% DoD, and 90% efficiency requires about 115.74 Ah.
Planning margin
Treat the result as a starting point. Battery aging, temperature, peak loads, inverter behavior, and limits on the chosen chemistry can reduce usable performance. A practical design review should include margin and manufacturer data.
Frequently asked questions
How do I estimate required battery capacity?
Multiply the connected load by the desired runtime, then divide by voltage, depth of discharge, and inverter efficiency.
Why is the answer in Ah instead of Wh?
The calculator reports nominal amp-hour capacity at the selected system voltage. Watt-hours can be found by multiplying Ah by volts.
Should I add practical margin?
Yes. The result is a planning estimate. Real systems may need additional margin for temperature, aging, load variation, conversion losses, and manufacturer limits.
Does the result guarantee a battery will work?
No. Confirm chemistry, voltage, continuous current, surge current, temperature range, and manufacturer requirements before selecting equipment.
Important assumptions
This is a planning estimate, not a guarantee of performance. Real equipment varies with temperature, age, power quality, duty cycle, wiring, and manufacturer specifications.