A portable heater used over winter needs a seasonal calculation. Enter its average electrical draw, the hours it actually heats and the number of days used in the year. Use your own tariff's unit rate in pounds per kWh. A daily standing charge remains a separate household cost; it does not become a saving each time the heater is switched off.
Formula
Energy in kWh = power in kW × hours of operation. A weekly schedule is multiplied by 365 ÷ 7; a monthly schedule uses days × 12, capped at 365. Cost = energy × price per kWh. With power and hours entered, the yearly schedule uses your exact number of operating days, without extrapolating the season to a full year.
Worked example
A winter heater without a full-year overestimate
Illustrative inputs, not a measured household or a quoted tariff: a 1,000 W heater operating at that draw for two hours on 120 days uses 1 × 2 × 120 = 240 kWh. Choose Year and 120 days. At an assumed 25p/kWh, enter £0.25, giving £60 per year and £5 per average calendar month. Entering 25 would make the price one hundred times too high.
What the result means
During a 30-day month with daily use, the same heater uses 60 kWh and costs £15. The £5 monthly output spreads the winter total across all twelve months. Reducing operation from two hours to one saves 120 kWh or £30 over those 120 days, provided the power draw stays the same. That does not show whether the resulting room temperature is suitable.
Electricity price · United Kingdom
Enter the unit rate including any applicable tax. Check your bill rather than adding a tax percentage automatically. Standing charges, export payments and subscription fees are separate. Ofgem information applies to Great Britain; Northern Ireland has its own regulator and market arrangements.
Assumptions and limits
A thermostat may stop and restart the heating element. If a suitable meter records 1.4 kWh over two hours, the average draw is 700 W, not necessarily the 1,000 W nameplate rating. Use equipment rated for the load and follow the heater's instructions. For a multi-rate tariff, calculate the use in each price window separately. Ofgem's bill guidance below explains unit rates and standing charges; Northern Ireland has a different regulatory framework.
Questions answered
Should I enter the appliance’s maximum power?
Measured average power is preferable when the appliance cycles or changes its output.
Does the cost include the whole electricity bill?
No. It estimates the variable energy charge at your chosen rate.
Data sources
- Ofgem · July–September 2026
- Ofgem: Understanding electricity and gas bills
- Ofgem: Price cap and standing charges explained
- Utility Regulator: Consumer information for Northern Ireland
Last reviewed: 2026-09-28