kWh calculator
Use the price from your bill if you know it.
Energy used
kWh = watts divided by 1,000, times hours. Amps times volts gives watts for a simple resistive load. Motors and electronics can draw less than the label on average.
The formulas
- Watts to kWh: kWh = watts divided by 1,000, times hours.
- Amps and volts to watts: watts = amps times volts.
- Cost: cost = kWh times your price per kWh.
The amps and volts formula is exact for simple heating elements such as a toaster or a space heater. Motors and electronics can have a power factor below 1, so their real draw is lower than amps times volts.
Quick conversions
| Power | 1 hour | 8 hours | 24 hours |
|---|---|---|---|
| 100 watts | 0.10 kWh | 0.80 kWh | 2.40 kWh |
| 500 watts | 0.50 kWh | 4.00 kWh | 12.00 kWh |
| 1,000 watts | 1.00 kWh | 8.00 kWh | 24.00 kWh |
| 1,500 watts | 1.50 kWh | 12.00 kWh | 36.00 kWh |
| 3,000 watts | 3.00 kWh | 24.00 kWh | 72.00 kWh |
Our guide to what a kWh is explains the unit with everyday examples.
Daily, monthly and yearly use
The calculator shows a 30 day month and a 365 day year. If a device only runs part of the year, such as a heater or a pool pump, multiply the daily figure by the days it actually runs. Utilities bill by billing period, which is often 28 to 33 days, so your own bill will not match a 30 day month exactly.
Frequently asked questions
How do I convert watts to kWh?
Divide the watts by 1,000 to get kilowatts, then multiply by the hours of use. A 1,500 watt device that runs for 2 hours uses 3 kWh.
How many watts is 1 kWh?
A kWh is a unit of energy and a watt is a unit of power, so the two are not the same thing. One kWh equals 1,000 watts used for one hour, or 100 watts used for ten hours.
How do I find kWh from amps?
Multiply amps by volts to get watts, divide by 1,000 for kilowatts, then multiply by the hours. A 15 amp circuit at 120 volts delivers up to 1,800 watts.
How many kWh does a house use per month?
The average US home used 874 kWh a month in 2025. Detached houses average about 1033 kWh and apartments in large buildings about 508 kWh.