The calculation
A watt is a rate and a kilowatt hour is an amount. An appliance rated at 1,000 W that runs for one hour uses one kWh, and that kWh is what your supplier charges for.
Cost per year = kWh per year × price per kWh
The share at full power matters for anything with a thermostat or a compressor. A fridge-freezer has a rated draw of 150 W only while its compressor runs, and the compressor runs for roughly a third of the day. Treating it as 150 W around the clock would overstate the cost by about three times.
Worked examples
All three use an electricity price of 0.28 per kWh.
| Appliance | Power | Use | Energy per year | Cost per year |
|---|---|---|---|---|
| Fridge-freezer | 150 W compressor | 24 hours a day, 35 percent at full power | 460 kWh | 128.77 |
| Portable electric heater | 2,000 W | 4 hours a day, 120 days, 60 percent at full power | 576 kWh | 161.28 |
| Electric kettle | 2,200 W | 18 minutes a day, every day | 241 kWh | 67.45 |
The kettle shows how a high wattage with short use still adds up. At 2,200 W it costs about 0.62 for every hour it is switched on, but it is on for only a fraction of an hour. The heater shows how a high draw over a heating season becomes a real number even with a thermostat cycling it down.
Where to find the wattage
Appliances carry a rating plate on the back, underneath or inside a door. It lists watts or amps and volts. If you only have amps, multiply by the voltage, which is 230 V in most of Europe. A 10 A heater at 230 V draws 2,300 W.
The best number comes from measurement. A plug-in energy meter costs little, sits between the appliance and the socket, and reports the kWh used over any period. Leave it on a fridge for a week, or on a television setup for a month, and you have a real figure that includes standby use.
Standby and always-on devices
Devices that never fully switch off, such as routers, set-top boxes and smart speakers, add up quietly. A 10 W device running all year uses 87.6 kWh. Enter 10 W, 24 hours and 365 days to see the cost. The saving from unplugging one of these is small, but a group of them can add up to a noticeable share of a low-use home.
Using it to find savings
Run the calculator for your five most-used appliances and compare. Heating, hot water, cooking and refrigeration usually dominate. Replacing a bulb saves little in euros compared with changing how long an electric heater runs, and the numbers make that visible. For lighting, see the LED savings calculator.
Frequently asked questions
Why is my figure different from the energy label?
Energy labels use standard test conditions. Your room temperature, door openings and load differ, so a measured figure is the more reliable one.
How do I enter a washing machine or dishwasher?
These are priced per cycle. Find the kWh per cycle on the label or in the manual, multiply it by 1,000 and enter the result as the watts, then set the hours to the number of cycles per day. A cycle that uses 0.9 kWh becomes 900 W for one hour.
What price should I use?
Use the unit rate from your bill, including VAT. Do not include the standing charge, because it does not change with use. The electricity bill calculator shows how the fixed part affects your overall price.
Does it work for appliances on a time-of-use tariff?
Yes. Enter the price that applies when you run the appliance.