From watts to kWh
An appliance's power is listed in watts (W) on the rating plate, in the manual or on the box. How much energy it uses depends on how long it runs. We measure that in kilowatt-hours (kWh): the energy used by a 1,000-watt appliance running for one hour. Your electricity bill charges per kWh.
kWh = power in watts × hours / 1,000
>
cost = kWh × price per kWh
A 2,000-watt kettle boiling for 3 minutes (0.05 hours) uses 2,000 × 0.05 / 1,000 = 0.1 kWh.
Worked examples
These examples use a rate of $0.20 per kWh; enter your own rate from your bill in the calculator.
- Tumble dryer, 2,000 W, 1 hour per load, 150 loads a year: 300 kWh, about $60 a year.
- LED bulb, 8 W, 5 hours a day, all year: 14.6 kWh, about $2.92 a year.
- Laptop, 50 W, 8 hours a day, 250 working days: 100 kWh, about $20 a year.
- Window air conditioner, 1,000 W, 6 hours a day, 60 summer days: 360 kWh, about $72 a year.
- Standby power of 5 W, day and night: 43.8 kWh, about $8.76 a year per device.
Power rating is not always actual use
The wattage on the rating plate is often the maximum power. Many appliances don't draw full power all the time:
- A fridge or freezer cycles on and off. Look at the annual kWh figure on the energy label (such as the EnergyGuide label in the US) rather than the wattage.
- A washing machine uses most energy heating water; a cold or 30 °C wash costs much less than a hot one.
- A TV or computer uses more with a bright screen or demanding games.
- A heat pump or induction hob adjusts its power constantly.
For exact figures, use a plug-in energy monitor or check your smart meter's app while the appliance runs on its own.
Which price per kWh?
Use the rate from your own bill, ideally the all-in price per kWh including taxes and delivery charges. Rates differ enormously between countries, states and contracts, and some plans charge different prices by time of day. With a time-of-use plan, running the dryer or dishwasher in off-peak hours can be noticeably cheaper.
Saving energy
The biggest users at home are usually appliances that produce heat (dryer, oven, electric heaters, water heater) and devices that are always on (fridge, freezer, router, standby power). Replacing an old appliance, washing colder or shorter and switching devices off properly can save a meaningful amount each year. Living in the Netherlands? The Dutch energy bill calculator estimates your whole household's electricity and gas costs.
Reading the label
Many appliances list their consumption in more than one way. A rating plate may show volts and amps instead of watts: multiply them to get watts (230 V × 10 A = 2,300 W in Europe, 120 V × 12.5 A = 1,500 W in the US). Energy labels give an annual kWh figure based on a standard usage pattern, which is often the most realistic number for fridges, freezers and TVs. For devices with a charger, such as phones and e-bikes, the charger's maximum output is only drawn while the battery is filling.
Frequently asked questions
How do I convert watts to kWh?
Multiply the power in watts by the number of hours and divide by 1,000. 1,500 W for 2 hours is 3 kWh.
How much does 1 kWh cost?
It depends on where you live and your plan. Check your electricity bill for the price per kWh and enter it in the calculator.
What is the difference between kW and kWh?
kW is power: how fast energy is used at a moment. kWh is energy: power multiplied by time. A 1 kW appliance running for 1 hour uses 1 kWh.
How much electricity does a fridge use?
Check the annual figure on the energy label. Modern fridges often use roughly 100 to 400 kWh a year, depending on size and model.
How can I measure actual use?
With a plug-in energy monitor between the plug and the socket, or through your smart meter's app.
Does standby power really add up?
Yes. A few watts per device seems small, but it runs all year. Ten devices at 3 watts each use about 263 kWh a year.
Last reviewed: 2026-10-06. Results are estimates for information only.