Electrical calculator

Amp Hours to Watt Hours Calculator

Convert battery charge capacity to nominal and usable energy, with an optional runtime estimate.

Convert battery capacity into stored energy

Amp-hours describe charge capacity, while watt-hours describe energy. Multiplying amp-hours by nominal voltage provides a convenient nominal-energy estimate. Applying a usable percentage can represent discharge limits, inverter losses, temperature effects, and a planning reserve in one combined factor.

Worked example

A 100 Ah, 12 V battery has 1,200 Wh, or 1.2 kWh, of nominal energy. At an 85% usable factor, the planning estimate is 1,020 Wh. A constant 100 W load would therefore run for about 10.2 hours in the simplified model.

Why actual runtime differs

• Battery voltage changes with state of charge and load; nominal voltage is a label, not a constant measurement.
• Battery chemistry, age, temperature, discharge rate, and manufacturer limits affect usable capacity.
• Inverters and wiring consume energy, and many devices cycle rather than draw a constant load.
• A conservative usable percentage can combine expected losses and reserve, but product-specific data is better.

Frequently asked questions

Can I add amp-hour ratings together?

For identical batteries in parallel, amp-hours add while nominal voltage stays the same. In series, voltage adds while amp-hours stay the same. The batteries and connection must be compatible and safely engineered.

Are watt-hours the same as watts?

No. Watts measure power at a moment; watt-hours measure energy over time. A 100 W load running for two hours uses 200 Wh in the ideal calculation.

What usable percentage should I choose?

Use the battery and system manufacturer’s limits. The default is only an example, not a recommendation for any chemistry or installation.

How to get a dependable result

• Use nameplate ratings or manufacturer specifications when they are available.
• Keep every input in the unit shown beside its field.
• Treat calculated values as planning estimates; real equipment, batteries, and conductors have losses, tolerances, and operating limits.

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Sources and review notes

The conversion follows SI power and energy relationships. Use the battery, charger, and inverter manufacturers’ documentation for safe operating limits.

Last reviewed September 2026. Results are educational estimates and do not replace product instructions, electrical codes, engineering review, or professional advice.