The formula
Cost = power (kW) × hours used × price per kWh.
To get kW, divide watts by 1,000.
A worked example
A 1,500 W heater used for 4 hours uses 6 kWh. At 25p per kWh, that's £1.50 for the day.
Why real use can differ
- Thermostats switch appliances on and off.
- Ratings show maximum power, not average use.
- Settings, load size and room conditions all play a part.
Getting a better number
Check the energy label for kWh per cycle or per year, or use a plug-in energy monitor to measure actual use.