# What size electric heater do I need? Sheet 05 · Heating and damp, whichfits.uk. A Markdown copy of https://whichfits.uk/electric-heater-size. The page itself has the working form and the Amazon.co.uk links: link to the page, not to this copy. 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. ## Work it out Worked example, with the inputs the sheet opens on: - What the room is: Living room - Length (m): 4 - Width (m): 3.5 - Ceiling height (m): 2.4 - Insulation: Average: double glazing, some loft insulation, cavity walls - Windows: Double glazed, nothing unusual - Outside walls: One outside wall **2 kW** 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.uk ## What 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. ## The 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. ## Oil-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. ## Bathrooms 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. ## Where to get it - Get a 650 W oil-filled radiator with a thermostat (the small one) on Amazon.co.uk. For a small bedroom or a box room. The thermostat does the rest. - Get a 1.5 kW oil-filled radiator on Amazon.co.uk. A medium bedroom, a small living room. - Get a 2 kW oil-filled radiator with a timer on Amazon.co.uk. The living room size. The timer means it is warm before you are. - Get a 2.5 kW oil-filled radiator on Amazon.co.uk. A big or badly insulated room. Above this you are into two heaters or a radiator. 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. ## Where 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](https://mcscertified.com/wp-content/uploads/2025/02/MIS-3005-D-2025-V1.0.pdf), 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](https://www.bestheating.com/info/btu-and-radiator-size-guide/), 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](https://www.traderadiators.com/heating-calculator), Trade Radiators. Single glazing +10%; two outside walls +5%, three +10% (read 17 October 2026) Data on this sheet last changed 17 October 2026. **Other sheets** - Sheet 06 [What size radiator do I need?](https://whichfits.uk/radiator-btu-calculator) - Sheet 08 [How much does this heater cost to run?](https://whichfits.uk/heater-running-cost) - Sheet 07 [What size dehumidifier do I need?](https://whichfits.uk/dehumidifier-size) --- whichfits.uk is written and published by Jaco Olwagen, with software doing the sums. Links to Amazon.co.uk earn the site a commission if you buy something, and that is how the lights stay on. Nothing here is bought, tested or handled: every number on every sheet comes from the documents named at the foot of that sheet, with the date they were read.