which fits?

Which battery, which bag, what size, how many. Look it up, then buy the right one once.

Sheet 05 · Heating and damp

What size electric heater do I need?

Your room is a box of cold air and the walls are winning. Every hour, heat leaks out through the glass, the brick, the gap under the door and the loft you have never been up into, and a heater has one job: put it back faster than it goes. That is a number. Not a feeling, not a lifestyle, a number in watts, and it comes from the size of the box and how badly the box leaks.

The shop will not work it out for you. The shop has a 2 kW fan heater with a sunrise on the box and it will sell you that for a bedsit or a barn. Put the room in below and get the wattage, then the next size up that is actually sold, then which kind of heater suits the room. Then buy one and shut the door.

1Work it out

The room

2.4 m is the usual modern ceiling. Victorian rooms run 2.7 to 3 m.

How well it holds heat
Insulation
Windows
Outside walls
2kW

33.6 cubic metres of living room held at 21 °C on a cold day needs about 1,848 W, so the next size up is 2 kW. An oil-filled radiator if the room is used all evening; a convector if you want it warm in ten minutes and off in an hour.

Get a 2 kW oil-filled radiator with a timer on Amazon.co.ukor search Amazon.co.uk for “2000W oil filled radiator timer”

2What the number means

The sum is the one the radiator trade publishes for when nobody has surveyed the house: the room’s volume in cubic metres, times a figure in watts per cubic metre for what the room is (a living room wants more than a bedroom, a bathroom more again), then a bit on for poor insulation, big or single-glazed windows and north-facing or exposed walls, and a bit off for a modern, well-insulated room. A draughty Victorian room needs about a third more than the same room in a new build, which is why ‘what size heater for a 4 by 4 room’ has no single answer and this sheet asks about your walls.

The figures assume the design day the trade sizes for: 21 degrees inside a living room, 18 in a bedroom, 22 in a bathroom, and about three below freezing outside, so the heater copes on the worst night in January and not the average one. Not sure about the insulation? Knock it down a notch. A heater that is too big just clicks off sooner.

3The 3 kW wall

A plug-in heater tops out at 3 kW, because that is what a 13 amp plug can carry, and most stop at 2 kW or 2.5. If the sum says more, the honest answer is two heaters on two sockets, or a radiator on the central heating, and the radiator sheet does that sum. Two 2 kW heaters on one extension lead is not an answer, it is a fire. One heater per wall socket, nothing on a four-way.

4Oil-filled, convector or fan: which for which room

An oil-filled radiator is a tank of oil with an element in it. Slow to warm, silent, and it keeps giving heat after the thermostat clicks off, so it is the bedroom heater and the all-evening living room heater. A convector heats the air and sends it up the wall: quick, quiet enough, cold the moment it stops. A fan heater is a convector with a hairdryer in it: warm in a minute, loud, dry as a bone. It is for the ten minutes you get dressed in, not for the evening.

None of them is more efficient than the others. Every electric heater turns all of its watts into heat, whatever the box says about ceramic, infrared or eco. The differences are how fast, how loud, and how long the heat hangs about.

5Bathrooms

Not with a plug. A bathroom heater has to be a fixed one with an IP rating, wired in, out of reach of the bath, and if that sounds like a job for an electrician, it is.

6Where to get it

Those links go to Amazon.co.uk and earn this site a commission if you buy. The price is the same either way, and nothing on this sheet changes because of it.

7Where the numbers come from

  1. MIS 3005-D Heat Pump Design, Issue 1.0: Table 1 internal design temperatures (from CIBSE Guide A) and Table 2 external design temperatures, MCS. Living room 21, bedroom 18, kitchen 18, hall 18, bathroom 22 °C; outside, London minus 3.0 °C (the temperature beaten only 0.4% of hours) (read 17 October 2026)
  2. BTU and radiator size guide: watts per cubic metre by room type, and the adjustments, BestHeating. Living room 50 to 60 W/m³, bedroom 40 to 50, bathroom 60 to 70, kitchen 50 to 60; poor insulation +15 to 25%, large windows +10 to 20%, north-facing +10%, well insulated −5 to 10%; watts × 3.41 = BTU/h. No hall figure is published, so the sheet uses the bedroom one (read 17 October 2026)
  3. Heating calculator, advanced adjustments, Trade Radiators. Single glazing +10%; two outside walls +5%, three +10% (read 17 October 2026)

Data on this sheet last changed 17 October 2026.