Calculating Electricity Use: From Watts to kWh and Cost
How many kilowatt-hours does an appliance use, and what does that cost on your tariff? Formula, calculation table and where to find the figures on your bill.
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Electricity is billed in kilowatt-hours (kWh). How many an appliance uses depends on just two quantities: its power in watts and how long it runs. With the price from your electricity bill, this becomes an amount in euros – without average prices that do not match your tariff.
Key takeaways
- Energy (kWh) = power (kW) × time (h). 1,000 watts for one hour make exactly 1 kWh.
- One kilowatt-hour equals 3.6 megajoules (1,000 W × 3,600 s).
- Cost = kWh × unit price; for a household, the standing charge is added.
- In Germany, the electricity bill must show your unit price, meter readings and consumption (section 40(2) Energy Industry Act, EnWG).
- The rating plate usually states the maximum power; actual consumption is measured with a plug-in meter or at the electricity meter.
The formula: power times time
Power is energy per time: 1 watt is 1 joule per second. Conversely, energy is power times time. Watt, joule, second and hour are legal units in Germany (Units Ordinance, Annex 1).
Consumption in kWh = power in watts × operating hours ÷ 1,000
The factor 1,000 turns watt-hours into kilowatt-hours. One kilowatt-hour is 1,000 watts × 3,600 seconds = 3,600,000 joules, i.e. 3.6 megajoules. Do not confuse kilowatts (kW, the power) with kilowatt-hours (kWh, the energy used).
Convert energy units
Convert between kWh, joules, calories and other units.
Calculation table: watts and hours in kWh
The table converts typical combinations of power and daily running time into kilowatt-hours. Use the values of your appliance – or use the calculator.
| Power | Use per day | kWh per day | kWh per year (365 days) |
|---|---|---|---|
| 5 W | 24 h | 0.12 | 43.8 |
| 10 W | 24 h | 0.24 | 87.6 |
| 60 W | 4 h | 0.24 | 87.6 |
| 100 W | 4 h | 0.4 | 146 |
| 1,000 W | 1 h | 1 | 365 |
| 2,000 W | 0.5 h | 1 | 365 |
Pure worked examples using the formula above, calculated with our electricity cost calculator; they are not measurements of specific appliances. The cost follows when you multiply the kWh by your unit price.
From kWh to euros: with your own price
For the cost of a single appliance, all you need is your unit price in cents per kWh. For the whole household, the standing charge is added, which is due regardless of consumption.
- Appliance: cost = kWh × unit price. An appliance running at 1,000 watts for one hour costs exactly one unit price.
- Household per year: annual consumption in kWh × unit price + 12 × monthly standing charge.
- Price per kWh including the standing charge: annual cost ÷ annual consumption. With low consumption it is much higher than the unit price.
- Always use gross prices, i.e. as they appear on the bill for you as a consumer.
Where is my price?
A German electricity bill must show the applicable prices, the opening and closing meter readings, the consumption in the billing period and a comparison with the previous year (section 40(2) nos. 3, 6 and 7 EnWG). Electricity tax and levies are listed separately (section 40(3) EnWG).
Electricity cost on your tariff
For one appliance or the whole household – with no preset price.
Measure consumption instead of guessing
- Read the meter and note the value together with the date.
- Read it again after a few days or weeks.
- Consumption per day = (new reading − old reading) ÷ number of days; multiplied by 365 it gives an estimate for the year.
- For a single appliance: a plug-in energy meter shows the actual power and adds up the kWh.
Rating plate is not the same as consumption
The rating plate states the power an appliance is designed for. Many appliances draw less in operation, and heating elements and compressors switch on and off. Use measured values if you need precision.
Convert power
Convert between watts, kilowatts and horsepower.
Frequently asked questions
How do I convert watts to kWh?
What does one hour of operation cost?
Why don't you state an average electricity price?
How do I find my annual consumption?
Sources
Every figure in this article comes from these primary sources or from the linked calculators, which are based on the same sources.
- Units Ordinance (EinhV), section 1 (legal units) (accessed September 30, 2026)
- Units Ordinance (EinhV), Annex 1 (watt, joule, hour) (accessed September 30, 2026)
- International Bureau of Weights and Measures: The International System of Units (SI Brochure) (accessed September 30, 2026)
- Energy Industry Act (EnWG), section 40 (content of electricity and gas bills) (accessed September 30, 2026)
Full-Stack Developer & Founder of HEADON.pro
Full-stack developer and founder of HEADON.pro. Developer of Rechnerzentrale.