Power and energy
Energy in kilowatt-hours
- where
- energy (kWh)
- power (W)
- duration (h)
Explain this equation
Works out energy in kilowatt-hours from a power rating and running time. This is the unit on the meter and the bill.
E is energy in kWh. P is the load’s real power, entered in watts and converted to kW. t is the running time in hours.
A kilowatt-hour is one kilowatt running for one hour. The calculator divides watts by 1,000 to get kW, then multiplies by hours.
Sense check: a 1,000 W heater for 2 hours uses 2 kWh. A 3 kW immersion for 30 minutes uses 1.5 kWh. One kWh is 3.6 million joules.
Watch out: it is kWh, not kW/h. Use average power for loads that cycle on and off, such as thermostatic heaters.
Electricity meters commonly record kilowatt-hours. Divide watts by 1,000, then multiply by hours of operation.
One kWh is 3.6 million joules. Keep the chosen unit system consistent from start to finish.
Enter your values to see the result.
Common mistake
Writing kW/h when you mean kWh.
Limits
Constant or average real power.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.