Use this oil-filled radiator running cost calculator to estimate electricity use and cost from your heater’s wattage, your unit rate and the time it spends heating. It includes an adjustable element-on percentage so you can compare continuous operation with thermostat-cycling scenarios.
The calculator is an estimate, not a meter reading or a tariff recommendation. Enter the unit rate from your electricity bill. The starting rate of 25p/kWh is simply an illustration; it is not described as today’s price cap or a current supplier price.
Oil-filled radiator running cost calculator
Enter your own tariff and appliance ratings
The 25p/kWh starting value is an illustration, not a current tariff quotation.
Enter your figures and select Calculate running cost.
How to enter useful figures
Heater power: enter the watts for the setting you are using, rather than automatically entering the maximum rating. A heater rated at 2,000W may have lower selectable settings; use the manual or appliance label to identify them. Do not enter kilowatts in the watts field: 2kW means 2,000W.
Hours used per day: enter the length of your heating session. Element-on time: enter the percentage of that session during which the heating element draws the entered wattage. Use 100% for a continuous full-power scenario. A value of 50% means half of the session, not half the thermostat dial or a guaranteed average.
Tariff: use pence per kWh, including VAT if that is how your bill states your unit rate. Enter 25 for 25p, rather than 0.25. Days: choose how many days use the same daily pattern. The seven-day result assumes seven identical days; it does not infer your weekly schedule.
The formula behind the calculator
Daily kWh = watts ÷ 1,000 × hours per day × element-on percentage ÷ 100.
Daily cost in pounds = daily kWh × tariff in p/kWh ÷ 100.
Multiply the daily result by the number of days for the period cost.
For example: 2,000W ÷ 1,000 × 4 hours × 50% = 4kWh per day. At 25p/kWh, that costs £1 per day, £7 for seven days and £30 for 30 identical days. If the element instead runs continuously for the four hours, those costs double.
| Power used | Continuous hourly cost at 25p/kWh | Four hours at 50% element-on |
|---|---|---|
| 700W | 17.5p | 35p |
| 1,000W | 25p | 50p |
| 1,500W | 37.5p | 75p |
| 2,000W | 50p | £1.00 |
| 2,500W | 62.5p | £1.25 |
Why your thermostat percentage is uncertain
A cold room may keep the element on for most of the warm-up period. Once the room reaches its target, the thermostat may cycle it on and off. A colder day, an open door, a higher target temperature or more draughts can increase the element-on time. The dial position alone does not tell you the percentage.
Try 100%, 75% and 50% to see the effect of different assumptions; do not choose the lowest result as a forecast. If you have a reliable energy measurement for a representative session, dividing its kWh by the hours gives average kW. You can then compare that average with the entered heating power, allowing for any other loads included in the measurement.
Does an oil-filled radiator’s residual heat cost nothing?
Warm oil and metal can continue releasing heat after the element stops drawing power. That energy was stored during warm-up. It is not an extra supply of free energy. Dimplex’s direct-acting heating guide explains the role of thermal mass and heat delivery.
What this estimate excludes
The calculation models the heating element at one selected wattage. It excludes small control or standby loads and assumes an unchanged tariff. It does not calculate a room’s heat loss, predict thermostat cycling or compare whole-home heating systems. The standing charge is excluded because it is generally payable whether this particular heater is used or not.
If your tariff changes by time of day, calculate each period separately and add the results. If you use different heat settings during a session, calculate those segments separately too. Do not interpret a monthly estimate as a fixed addition to your bill regardless of weather or routine.
Use the result to plan your heating
Compare realistic occupied hours and a comfortable thermostat setting before buying a new heater. Read our guide to 2kW electric heater running costs for another worked example, or our oil-filled radiator versus fan heater comparison when choosing a heating format. More budgeting guides are in Energy & Running Costs guides.
Follow the heater manual and Electrical Safety First’s portable heater advice. Never cover a portable heater or use it to dry clothing; turn it off before leaving it unattended or going to sleep.
Can I use this calculator for a convector heater?
Yes, the same arithmetic applies to a resistance heater when you enter its actual wattage and operating pattern. It does not establish that two different heaters will achieve the same comfort in your room.
