Energy fundamentals
Watts vs. Watt-Hours
Learn the difference between power and energy and avoid a common battery-sizing mistake.
The essential idea
Watts measure the rate at which a device uses or produces energy. Watt-hours measure an amount of energy over time.
How to apply it
A 100-watt device running steadily for three hours uses 300 watt-hours. The same 300 Wh could supply 300 W for one ideal hour, before practical losses.
A practical decision
Use watts when checking whether an inverter can handle a load. Use watt-hours when estimating runtime or daily energy.
Use this in a calculator
Browse all home energy calculators and replace the examples with measured or manufacturer-provided values.
Last reviewed: August 6, 2026