What this calculator does
A watt-hour is a unit of energy: one watt of power sustained for one hour. This Wh calculator multiplies the power draw of a device by how long it runs to give the total energy used, which is the figure printed on battery capacity labels and the raw quantity behind every electricity bill, before any price is applied.
It is deliberately simpler than a running-cost calculator. There is no price per kilowatt-hour here, just power and time in, energy out. That makes it the right tool when the question is how much energy a battery holds or a device draws over a session, rather than what that energy will cost.
The formula
Multiply the power draw in watts by the running time in hours to get watt-hours. Dividing by 1,000 converts that into kilowatt-hours, the unit most batteries and appliance ratings are actually quoted in once the numbers get larger.
| Term | Meaning |
|---|---|
| Wh | Watt-hours: power in watts × time in hours. |
| kWh | Kilowatt-hours: watt-hours ÷ 1,000, the unit used on power bills and larger battery packs. |
| W | Watts, the rate of energy use printed on most appliance labels. |
The inputs explained
| Field | What to enter |
|---|---|
| Power draw (W) | The power draw of the device, in watts, usually printed on a label or in its specifications. |
| Running time (hours) | How long the device runs for, in hours. Use a decimal for part hours, such as 1.5 for 90 minutes. |
When to use it
Sizing a battery or power bank
Battery capacity is usually quoted in watt-hours or milliamp-hours at a stated voltage. Working out how many Wh a device needs over a session shows whether a given battery can actually run it for that long.
Comparing appliance energy use
Two appliances with different wattages and run times can be compared fairly once both are converted to watt-hours or kilowatt-hours for the same period.
Estimating solar or generator load
Off-grid and generator setups are planned around total watt-hours needed per day, adding up every device rather than looking at wattage alone.
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 use changes with running time at a fixed power draw
A steady 60 W load, run for a range of durations.
| Running time | Energy used | Same, in kilowatt-hours |
|---|---|---|
| 0.5 h | 30.00 Wh | 0.0300 kWh |
| 1 h | 60.00 Wh | 0.0600 kWh |
| 2 h | 120.00 Wh | 0.1200 kWh |
| 4 h | 240.00 Wh | 0.2400 kWh |
| 8 h | 480.00 Wh | 0.4800 kWh |
| 24 h | 1,440.00 Wh | 1.440 kWh |
How energy use changes with power draw over a fixed running time
A fixed 3-hour session, across a range of appliance wattages.
| Power draw | Energy used | Same, in kilowatt-hours |
|---|---|---|
| 10 W | 30.00 Wh | 0.0300 kWh |
| 25 W | 75.00 Wh | 0.0750 kWh |
| 60 W | 180.00 Wh | 0.1800 kWh |
| 100 W | 300.00 Wh | 0.3000 kWh |
| 500 W | 1,500.00 Wh | 1.500 kWh |
| 1500 W | 4,500.00 Wh | 4.500 kWh |
Questions
What is the difference between watts and watt-hours?
Watts measure the rate of energy use at any given moment, like speed. Watt-hours measure the total energy used over time, like distance travelled. A 100 W bulb draws 100 W whether it has been on for a minute or a day, but the watt-hours used keep climbing the longer it stays on.
How do I convert Wh to cost?
Divide watt-hours by 1,000 to get kilowatt-hours, then multiply by the price charged per kilowatt-hour. For a full running-cost breakdown including a daily and yearly total, use the electricity running cost calculator.
How is Wh different from mAh on a battery label?
Milliamp-hours (mAh) measure charge, not energy, and only convert to Wh once the battery voltage is known: Wh = mAh × voltage ÷ 1,000. Two batteries with the same mAh rating but different voltages store different amounts of energy.
Why does my device use less energy in practice than its rated wattage suggests?
Rated wattage is often a peak or maximum figure, while many devices cycle on and off or run below full load for parts of their operation, so the average power draw, and therefore the real watt-hours used, can be noticeably lower than the label implies.
For a full cost breakdown once a price per kilowatt-hour is known, see the electricity running cost calculator.