Energy and vehicle · Electricity
Calculate what an electrical appliance costs
Power, running time and the price of a kilowatt-hour are enough. The tool returns consumption and spend per day, per month and per year.
Calculate
How the calculator works
A power in watts becomes an energy as soon as you multiply it by a duration. Divide the watts by 1,000 to get kilowatts, multiply by the hours of daily use, and you have kilowatt-hours per day. Cost follows by multiplying by the price per kWh. A month counts 30 days and a year 365, so the year is not exactly twelve times the month, and that is deliberate.
Formula and method
kWh/day = (W ÷ 1000) × hours
A power becomes an energy once multiplied by a duration.
cost/day = kWh/day × price per kWh
The all-in rate on the contract.
month = 30 days ; year = 365 days
Stated conventions, to avoid false precision.
The label rating is a maximum. A thermostat-controlled heater, a fridge or an air conditioner cycle on and off: their real running time is shorter than the time they are switched on. Enter effective operating time, not plug-in time. Standby draw, small but permanent, is not modelled here.
Worked example
A 2,000 W heater, 3 h a day, at 0.25 per kWh
power = 2000, time = 3, price = 0.25.
- kWh/day: 2 × 3 = 6 kWh.
- Cost/day: 6 × 0.25 = 1.50.
- Month: 1.50 × 30 = 45.
- Year: 6 × 365 × 0.25 = 547.50.
1.50 a day, 45 a month, 547.50 a year.
A 9 W LED bulb on for 5 h a day
power = 9, time = 5, price = 0.25.
- kWh/day: 0.009 × 5 = 0.045 kWh.
- Year: 0.045 × 365 × 0.25 ≈ 4.11.
About 4 a year: lighting barely registers next to heating.
Input notes
- Appliance power
- The wattage printed on the appliance. Convert kilowatts to watts before entering.
- Time in use
- Actual running time, not the time it stays plugged in.
- Price per kWh
- The all-in price of a kilowatt-hour on your contract, standing charge excluded.
Assumptions and limits
Assumptions
- The appliance draws its rated power for the whole time entered.
- A month is 30 days and a year 365 days.
- The price per kWh is constant and excludes the standing charge.
Limits
- No standby draw, no thermostat cycling, no start-up surge.
- Standing charges and fixed taxes are not counted.
- Peak and off-peak rates need two separate calculations.
How to read the result
Ranking the big items matters more than precision to the cent. A high-wattage appliance running for hours — heating, water heater, tumble dryer — dominates the bill, while LED lighting or a phone charger cost a few units a year. If your bill surprises you, look first for something above 1,000 W that runs several hours a day.
Common mistakes
Confusing kW with kWh.
The kW is instantaneous power; the kWh is energy used over time.
Entering plug-in time for a thermostatic appliance.
A regulated heater only heats part of the time. Estimate the real duty cycle.
Expecting the year to be twelve times the month.
The month uses 30 days and the year 365, five days more.
Comparison
| Appliance | Power | Use | Cost per year |
|---|---|---|---|
| LED bulb | 9 W | 5 h/day | ≈ 4 |
| Laptop | 60 W | 8 h/day | ≈ 44 |
| Portable heater | 2,000 W | 3 h/day | ≈ 548 |
| Tumble dryer | 2,500 W | 1 h/day | ≈ 228 |
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Frequently asked questions
What does a 2,000 W heater cost?
About 1.50 per 3 hours.
At 0.25 per kilowatt-hour, 2,000 W for 3 hours uses 6 kWh, so 1.50. Over a winter month at that rate it is 45, and 547 if the same use ran all year.
Is standby included?
No.
The calculation only uses the running time you enter. To estimate standby, run it again with the standby power, often 0.5 to 3 W, over 24 hours.
Should I include the standing charge?
No, only the price per kWh.
The standing charge is fixed and independent of this appliance. Add it separately if you are rebuilding the whole bill.
Author and update
Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.
- NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
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