Watt-hours measure a total amount of energy — this calculates it from a device’s power rating and how long it runs, useful for sizing batteries or power banks.
How it works
Watts are multiplied by hours to give watt-hours; dividing by 1,000 converts that to kilowatt-hours.
What this does not include
This does not include a dollar cost — for the cost of running an appliance at a given electricity rate, use this site’s electricity cost calculator instead.
How to use this calculator
- Enter the power rating in watts and the time in hours.
A worked example
A 60-watt device running for 5 hours: 60 × 5 = 300 watt-hours, or 0.3 kilowatt-hours.
A 10-watt device running for 24 hours: 240 watt-hours.
What the variables mean
| Variable | Meaning |
|---|---|
| Watts | Power draw of the device |
| Hours | How long it runs |
Edge cases worth knowing
Watt-hours measure energy, not power — a small device running a long time and a large device running briefly can use the same total energy despite very different power ratings.
Electricity bills are typically priced in kilowatt-hours, not watt-hours — dividing by 1,000 converts to the unit that actually shows up on a utility bill.
Frequently asked questions
What’s the difference between watts and watt-hours?
Watts measure a rate of energy use at a single moment; watt-hours measure a total amount of energy used over time.
Why would I need to know watt-hours?
It’s the standard way battery capacities and power banks are rated, so it’s useful for sizing one to a device’s needs.
How does this relate to kilowatt-hours on a power bill?
A kilowatt-hour is simply 1,000 watt-hours — this calculator shows both units side by side.
