kWh calculator
One kilowatt-hour runs a 1,000 W heater for an hour, a 100 W bulb for ten hours, or a 10 W LED strip for four days. It boils about ten kettles, or drives an electric car roughly six kilometres. It is the unit every energy bill is denominated in, and putting an appliance into it is the fastest way to see whether it is worth worrying about.
Kilowatt-hours are watts times hours divided by a thousand. A 1,000 W heater running three hours a day uses 3 kWh daily, 91 kWh a month and 1,096 kWh a year — about £307 at 28p per unit.
How to calculate kWh
The gap between rated wattage and real consumption is where estimates go wrong. A fridge is rated at its compressor power but runs maybe a third of the time. A washing machine draws most of its energy in the heating phase and very little for the rest of the cycle. A television rated at 120 W might average 60. For anything that cycles or modulates, the rating plate is an upper bound rather than an average, and a plug-in energy monitor left on for a week gives a truer figure than any calculation.
Questions
Multiply watts by hours of use and divide by 1,000. A 500 W device for 2 hours uses 1 kWh.