Oil-filled radiators cost the same per hour as any electric heater at the same wattage. Over a full day on a thermostat they use 15 to 25 per cent less electricity because the oil retains heat after the element switches off. This reduces the number of times the element has to cycle on.

The Answer: Same Rate Per Hour, Lower Cost Per Day

Every electric heater — oil-filled, convector, fan, or panel — converts electricity to heat at 100 per cent efficiency. A 1,500W oil-filled radiator uses exactly 1.5kWh per hour of operation, as does a 1,500W convector heater. At the UK Ofgem price cap rate of 27.69p per kWh (January 2026), both cost 41.5p per hour while running.

The difference is in how many hours each runs. An oil-filled radiator retains heat in its oil reservoir for 20 to 30 minutes after the element switches off. A convector heater loses its heat within minutes. The oil-filled model keeps the room warm between cycles using the heat stored in the oil reservoir. The convector cannot do this and must run its element more often to maintain the same temperature.

Over a typical eight-hour heating session in a well-insulated UK room, an oil-filled radiator on a thermostat operates its element for roughly five to six hours. A convector maintaining the same temperature operates for six to seven hours. That difference compounds across a full heating season.

Running Cost Comparison (UK 2026)

These figures use the Ofgem price cap rate of 27.69p per kWh, current as of January 2026.

These are maximum figures — the actual cost when running on a thermostat is lower, because the thermostat cuts the element when the target temperature is reached. An oil-filled radiator reaches that cut-off point and stays there longer, reducing actual element-on time relative to a convector at the same wattage.

Why Oil-Filled Radiators Use Less Electricity Over Time

Diathermic oil — the sealed fluid inside an oil-filled radiator — has a high specific heat capacity. It absorbs a large amount of energy during the heating phase and releases it slowly. Once the radiator reaches operating temperature, the oil continues to radiate warmth into the room for 20 to 30 minutes after the element switches off, keeping the thermostat satisfied without drawing power.

A convector heater contains no thermal mass. When the element switches off, heat output stops within seconds. The room temperature drops more quickly, the thermostat calls for heat sooner, and the element switches on again. This shorter off-cycle means the element runs for more total hours per day.

In a room with reasonable insulation, this mechanism reduces oil-filled radiator element-on time by 15 to 25 per cent compared with a convector at the same thermostat setting. The colder the room (lower insulation, single glazing, older property), the greater the saving, because the oil's residual heat displaces more cycles.

When Other Electric Heaters Are Cheaper to Run

Oil-filled radiators provide their running cost advantage only in sustained heating sessions. Two scenarios exist where other heaters are more cost-effective:

• Sessions under two hours. An oil-filled radiator spends 10 to 15 minutes of its running time heating the oil before it reaches operating temperature. For a short burst of warmth, a convector or fan heater delivers full output immediately and can be switched off sooner. The oil-filled radiator would still be warming up as the convector finishes the job.
• Rooms you leave immediately after heating. The oil's residual heat is wasted if no one is in the room to benefit from it. A convector heats the air quickly and you leave — there is no advantage to slow heat release if the room is empty.

For a full comparison of both heater types with specific UK product recommendations, see our convector heater vs oil-filled radiator guide.

Best Oil-Filled Radiators for Low Running Costs

A precise thermostat is the single most important factor for minimising running costs in an oil-filled radiator. A thermostat that holds temperature within ±0.5°C cycles the element far less than a basic rotary dial that swings ±3–4°C. These models provide reliable thermostat control at UK price points:

Frequently Asked Questions

Are oil-filled radiators expensive to run?

No more than any other electric heater at the same wattage. A 1,500W oil-filled radiator costs 41.5p per hour at the January 2026 Ofgem rate of 27.69p/kWh. Over a full day on a thermostat, it typically costs 15 to 25 per cent less than a convector maintaining the same room temperature, due to oil heat retention between thermostat cycles.

What is the cheapest electric heater to run in the UK?

All electric heaters convert electricity to heat at 100 per cent efficiency, so running cost per hour is identical at the same wattage. The cheapest to run over a full day is an oil-filled radiator on a good thermostat, because the oil's heat retention reduces total element-on time. Infrared heaters are also efficient for targeted, short-duration heating of a person rather than a room.

How many units does a 1,500W oil-filled radiator use per hour?

A 1,500W oil-filled radiator uses 1.5 units (kWh) per hour of element operation. At the January 2026 Ofgem rate this costs 41.54p. On a thermostat, the element does not run continuously — in a well-insulated room it typically runs for 50 to 70 per cent of total on-time, reducing actual consumption to approximately 0.75 to 1.05 kWh per hour of clock time.

Are oil-filled radiators more efficient than fan heaters?

Both convert electricity to heat at 100 per cent efficiency, so neither is more efficient in the engineering sense. Oil-filled radiators are more effective for sustained room heating because the thermal mass reduces total element runtime. Fan heaters are more effective for rapid heat delivery in a single short session. The cheapest option depends on session length: fan heaters win for sessions under 90 minutes; oil-filled radiators win for sessions over two hours.

Can I leave an oil-filled radiator on all night?

Yes. Oil-filled radiators with a thermostat and overheat protection are designed for extended use including overnight operation. Set the thermostat to 16°C for an adult bedroom or 18°C for a child's room overnight. The thermostat controls element cycling automatically, preventing overheating and reducing electricity consumption during the night hours when less top-up heat is required.

Do oil-filled radiators dry the air?

Less than fan heaters or convectors. Oil-filled radiators heat the air gradually through radiant and convective heat from the fins, rather than blasting heated air through an element at high temperature. The more gentle heat transfer reduces moisture evaporation from the air. People who find fan heaters aggravate dry skin or sinuses often find oil-filled radiators more comfortable for long sessions.