The sum for your own heater
Every electric heater turns one kWh of electricity into one kWh of heat, so the arithmetic is the same for a fan heater, an oil filled radiator or a panel heater. Only the wattage changes.
Working out an hourly cost
- Find the wattageIt is on the label or the box, for example 2,000 W.
- Turn it into kWDivide by 1,000. A 2,000 W heater is 2 kW.
- Multiply by the unit price2 kW times 26.32p is about 53p for an hour flat out.
- Allow for the thermostatOnce the room is up to temperature the heater cycles, so a well sized one may run half the time.
Cost per hour by outside temperature
What really sets the bill is how much heat the room loses, and that depends on how cold it is outside and which roof sits overhead. We worked these from the heat loss of the model room in the Comfort Calendar: roof, three glass walls, dwarf wall and air leaks, with the room held steady at 19°C.
| Outside | Polycarbonate roof | Panel roof |
|---|---|---|
| 10°C | 0.8 kW, about 21p | 0.5 kW, about 13p |
| 5°C | 1.3 kW, about 34p | 0.8 kW, about 21p |
| 0°C | 1.7 kW, about 45p | 1.1 kW, about 29p |
| Minus 5°C | 2.2 kW, about 58p | 1.4 kW, about 37p |
Why the first half hour costs more
The table is for holding a temperature. Getting there takes extra, because the heater has to warm the air, the frame, the furniture and the top of the floor before the room feels right. A 2 kW heater running flat out for the first 30 minutes uses 1 kWh, about 26p, before it settles down.
So two short sessions a day can cost more than one longer one. If you use the room in the morning and again in the evening, a low background setting in between is sometimes cheaper than letting it go cold.
What it adds up to over a year
Our model holds 19°C through every waking hour, 8am to 10pm, whenever the room would otherwise be colder, across two years of real weather for 104 towns. That is a heavy user. Most people heat a conservatory for a few hours a day, so treat these as the top of the range.
- £433
- a year to heat a polycarbonate roofed room
- £368
- a year under an insulated panel roof
- £65
- saved, which is 15% of the bill
Making each hour cheaper
- Heat pump. An air to air unit gives roughly three times as much heat as it uses, so each figure in the table drops to about a third.
- Timer and thermostat. Warm the room for when you sit down, not all afternoon.
- Curtains at dusk. Bare glass loses heat to the night sky and makes you feel colder at the same air temperature.
- The roof. It is 41% of the heat loss under polycarbonate. The saving is real but modest, and the bigger win is summer, as the summer cooling answer shows.
Our guide to the best heaters for a conservatory compares the options, and the prices page has roof costs by size.
