How do you calculate watt-hours?
Watt-hours are calculated by multiplying power in watts by time in hours:
Wh = W × h
Example: 100W × 5 hours = 500Wh. A 100W device running for half an hour uses 50Wh.
Battery capacity conversions
These are nominal energy conversions. They do not account for depth of discharge, inverter losses, battery condition or a planning reserve. Use the battery runtime calculator for a more realistic operating estimate.
Watts, watt-hours and amp-hours
A watt is a rate of power. A watt-hour is an amount of energy. Amp-hours describe electric charge and only convert to energy when voltage is known. That is why a 100Ah battery at 12V stores half the nominal energy of a 100Ah battery at 24V.
Need the reverse conversion? Use the amp-hour calculator to convert watt-hours to amp-hours.
How to use the calculator
- Choose the conversion that matches the information you have.
- Enter power, time, voltage or capacity in the displayed fields.
- Read the result and formula. Change any input to update it instantly.
The runtime mode is ideal and theoretical: Wh ÷ W. For real batteries, usable capacity and conversion efficiency reduce operating time.
Watt-hour calculator FAQ
How do you calculate watt-hours?
Multiply power in watts by time in hours: Wh = W × h.
What is the difference between watts and watt-hours?
Watts measure power at a moment in time. Watt-hours measure energy used or stored over time.
How many watt-hours are in a 100Ah battery?
Multiply amp-hours by nominal voltage. A 12V 100Ah battery is 1,200Wh, while a 12.8V 100Ah battery is 1,280Wh nominally.
How do I convert Ah to Wh?
Multiply amp-hours by voltage: Wh = Ah × V.
How do I convert mAh to Wh?
Divide milliamp-hours by 1,000, then multiply by voltage: Wh = (mAh ÷ 1,000) × V.