What this calculator does
Watts measure power: the rate energy is being used at any given instant. Watt-hours measure energy: how much was actually used over some stretch of time. This watts to watt hours calculator supplies the missing piece, converting watts to watt hours once you add how long the device actually ran.
A 100-watt device left running for 5 hours does not use 100 watts of anything; it uses 500 watt-hours of energy. Watts alone say nothing about total energy use without a time attached, which is exactly why watt-hours, not watts, are the figure that turns up on an electricity bill.
The formula
To calculate watt hours, multiply the power in watts by the running time in hours: watt-hours = watts × hours. Dividing that figure by 1,000 converts it to kilowatt-hours, the unit electricity is normally billed in, and multiplying the kilowatt-hours by a price per kilowatt-hour estimates the cost.
| Term | Meaning |
|---|---|
| Watts (W) | Power: the rate energy is used, at a single instant. |
| Watt-hours (Wh) | Energy: power sustained over time, watts × hours. |
| Kilowatt-hours (kWh) | Watt-hours divided by 1,000, the unit used on most electricity bills. |
The inputs explained
| Field | What to enter |
|---|---|
| Power (W) | The power rating of the device, in watts, usually printed on it or its power supply. |
| Time running (hours) | How many hours the device actually ran for. |
| Electricity price (0 to skip cost estimate) ($/kWh) | Optional. Your electricity price per kilowatt-hour, to estimate the cost of that energy use. Leave at 0 to skip the cost estimate. |
When to use it
Working out what a device actually costs to run
A device’s wattage alone does not say much about its running cost; multiplying by the hours it is actually used, and then by an electricity price, turns watts into a dollar figure.
Sizing a battery or power bank
Batteries and power banks are commonly rated in watt-hours or milliamp-hours. Converting a device’s watts and expected running time into watt-hours shows whether a given battery can actually cover it.
Comparing two appliances with different power ratings
A lower-wattage appliance run for longer can use more total energy than a higher-wattage one run briefly; converting both to watt-hours puts them on the same footing.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How energy used scales with running time
A fixed 100-watt device, run for a range of durations.
| Running time | Energy used | In kilowatt-hours |
|---|---|---|
| 1 hours | 100 Wh | 0.100 kWh |
| 2 hours | 200 Wh | 0.200 kWh |
| 5 hours | 500 Wh | 0.500 kWh |
| 8 hours | 800 Wh | 0.800 kWh |
| 12 hours | 1,200 Wh | 1.200 kWh |
| 24 hours | 2,400 Wh | 2.400 kWh |
How energy used scales with power at a fixed running time
A fixed 5-hour running time, across a range of device wattages.
| Power | Energy used | In kilowatt-hours |
|---|---|---|
| 10 W | 50 Wh | 0.050 kWh |
| 25 W | 125 Wh | 0.125 kWh |
| 60 W | 300 Wh | 0.300 kWh |
| 100 W | 500 Wh | 0.500 kWh |
| 250 W | 1,250 Wh | 1.250 kWh |
| 1000 W | 5,000 Wh | 5.000 kWh |
Questions
How do I calculate watt hours from watts?
Multiply the power in watts by the number of hours it runs for: watt-hours = watts × hours. A 60-watt bulb left on for 10 hours uses 600 watt-hours.
What is the difference between watts and watt-hours?
Watts measure the rate of energy use at one moment, like speed. Watt-hours measure the total energy used over time, like distance travelled. The same wattage can produce very different watt-hour totals depending on how long it runs.
How do watt-hours relate to kilowatt-hours on my electricity bill?
A kilowatt-hour is 1,000 watt-hours. Electricity bills are priced in kilowatt-hours because watt-hours are usually too small a unit to be convenient for household-scale energy use over a billing period.
Can I convert watt-hours back to an average power draw?
Yes, by reversing the calculation: divide the watt-hours by the number of hours to get the average watts. This calculator is set up to go from watts and hours to watt-hours, which is the more common direction the question is asked.
For converting between watts and amps at a given voltage, see the watts to amps calculator. To size a generator for a set of appliances by their running and surge wattage, use the generator sizing calculator.