Electrical Energy Calculator
Convert power and time into total energy consumed.
Formula
E (kWh) = P (kW) × t (h)
Inputs
- Power (W)
- Duration (hours)
What it does
Converts power and time into energy in kilowatt-hours — the unit on your electricity bill.
Where it's used
Estimating appliance running costs and battery or generator energy needs.
How to use it
- Enter the power in watts.
- Enter how long it runs, in hours.
- Multiply the kWh result by your tariff for the cost.
The formula explained
- E
- P(kW) × t(h), in kWh
- 1 kWh
- one kilowatt for one hour
Worked example
A 2 kW heater running for 1½ hours.
Inputs:
- Power: 2000 W
- Duration: 1.5 hours
The calculator gives:
- Result: 3 kWh
Tips and common mistakes
- Use the average power; thermostats switch heaters on and off.
- Standby loads add up over a month.
- For cost over a period, see the DC Power calculator.
Questions people ask
How do I turn kilowatt-hours into a cost?
Multiply by your tariff. The 2 kW heater running 1.5 hours uses 3 kWh, which costs 24 at a tariff of 8 per kWh.
How much does standby power cost over a year?
A device left on standby at 5 W uses 5 × 24 × 365 = 43.8 kWh a year. Ten such devices use as much as a 2 kW heater running for over 200 hours.
How many joules are in a kilowatt-hour?
One kilowatt-hour is 3.6 million joules (3.6 MJ), because 1 W is 1 J per second and an hour has 3 600 seconds.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.