kWh Calculator — Energy Use and Cost
Turns a wattage and a run time into kilowatt-hours and money. Use the rating plate figure for power, and your tariff for the price.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
kWh = watts × hours ÷ 1,000 cost = kWh × price per kWh
A kilowatt-hour is exactly what it says: one kilowatt drawn for one hour. The only trap is the factor of a thousand, which is why a 1,500 W heater run for an hour uses 1.5 kWh rather than 1,500.
What to watch out for
Two things throw the answer out more than the arithmetic does:
- Rated power is not average power. A fridge is rated for its compressor running, but it only runs perhaps a third of the time. Anything thermostatically controlled — fridges, heaters, kettles, ovens — uses far less than the plate suggests.
- Standby adds up. A few watts continuously is about 9 kWh a year per watt. Ten devices at 3 W each is 260 kWh, which is more than most people's fridge.
For an exact figure, a plug-in energy monitor over a week beats any calculation, because it measures the duty cycle you actually have.
Frequently Asked Questions
How do I calculate kWh from watts?
Multiply watts by hours and divide by 1,000. A 1,500 W heater for 3 hours uses 4.5 kWh.
How much does it cost to run a 1,500 W heater?
1.5 kWh per hour, so at 25p or 25 cents per kWh it costs about 37.5 per hour. Three hours a day for a winter month is roughly 135 kWh, or around 34 in the same currency.
How many watts is 1 kWh?
The question mixes two different things. A watt is a rate; a kilowatt-hour is an amount of energy. 1 kWh is 1,000 watts sustained for one hour, or 100 watts for ten hours — both are the same energy.