What size electric heater do I need for a room?

Sizing a plug-in heater is a rougher calculation than sizing a radiator, because a plug-in heater is almost always doing a different job: taking the edge off one room, quickly, rather than holding a whole house at temperature. But the arithmetic still matters, because the two ways of getting it wrong are not symmetrical. An oversized heater with a working thermostat simply reaches temperature and cycles off, so it costs little more. An undersized one runs continuously at full power, never reaches the set temperature, and costs the maximum the whole time.

Safety

What you'll need

• A tape measure
• An honest assessment of the room's insulation and glazing
• The room's existing radiator output, if it has one (optional)

Step by step

1. Measure the room in three dimensions.Length times width times height, in metres, giving cubic metres. Ceiling height matters and is the dimension people leave out — a Victorian room with a three-metre ceiling needs noticeably more than a modern one of the same floor area.
2. Choose a watts-per-cubic-metre figure for how the room actually performs.About 40 for a well insulated modern room with double glazing and one external wall. About 55 for an average room in an older house. About 70 for a conservatory, a room over a garage or a passage, one with large single-glazed windows, or one with two or more exposed external walls.
3. Multiply, then round to a size that is actually sold.Plug-in heaters come in a small number of sizes: commonly 400W to 800W panel and oil-filled models, then 1kW, 1.5kW, 2kW and 3kW. Round up to the nearest of those rather than agonising over the figure.
4. Check the 3kW ceiling.A 13A socket supports 3kW and no more. If your calculation gives more than that, a plug-in heater is the wrong tool for the room and the answer is the central heating, a correctly sized radiator, or a fixed heater on its own circuit installed by an electrician.
5. Prefer a thermostat over exact sizing.A slightly oversized heater with a thermostat reaches temperature and cycles off, using only what the room loses. A precisely sized heater without one runs flat out regardless. If you have to choose between a bigger heater with a thermostat and a smaller one without, take the thermostat.
6. Match the type to how the room is used, once the size is settled.A fan heater for short bursts in a bathroom or a home office you occupy for an hour. A convector or oil-filled model for sustained background heat in a room you sit in all evening. A radiant or infrared heater for warming a person quickly in a space too big or too draughty to heat at all, such as a workshop or a garage.
7. Reduce the requirement before buying, where you can.Draught-proofing the door and window, closing curtains at dusk and putting a rug on a cold floor all lower the number this calculation produces. They are one-off costs against a permanent running cost, and they should come first.
8. For a room that needs heating every day, check the central heating instead.Gas is typically much cheaper per kilowatt hour than electricity. If the boiler can heat that room via its own radiator and thermostatic valve, running it there and turning other radiators down is usually cheaper than any electric heater. Plug-in heating earns its place where the boiler cannot reach or is not worth firing.
9. Never solve an undersized heater with an extension lead and a second one.Two heaters on one socket via an adaptor exceeds what that socket and its flex are designed to carry. If one 3kW heater is not enough for the room, the answer is not two.

Tips

• Ceiling height is the dimension most often forgotten and the one that most changes the answer in an older house.
• A conservatory almost never sizes sensibly on this calculation, because the glazing loses heat faster than a plug-in heater can supply it. Insulating the roof changes the arithmetic far more than a bigger heater does.
• If a room already has a radiator, its output in watts is a good sanity check — the heating designer did a more careful version of this calculation when the system was installed.
• A timer plus a thermostat is worth more than any extra kilowatt. Heating a room for the hour before it is used costs a fraction of heating it all day.

Common mistakes

• Buying the smallest heater to save money — An undersized heater runs continuously at full power and never reaches temperature, so it costs the maximum for a room that still feels cold. Oversized with a thermostat costs less, not more.
• Leaving ceiling height out of the calculation — Volume, not floor area, is what has to be heated. A three-metre ceiling adds twenty percent over a 2.5-metre one for the same floor plan.
• Running a heater from an extension lead — Heaters are a sustained near-maximum load. Extension leads, reels and multi-way adaptors overheat under it, and this is one of the most common causes of domestic electrical fires.

Questions people ask