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This page explains how to calculate how much heat a room needs. For how much fuel will be used, go to the Fuel Cost Page.

If a room were perfectly insulated, it would need no heating - its temperature would rise to the body temperature of the people inside. Rooms only need heating because they lose heat to the outside world, so that the function of a heating appliance is to replace the lost heat.

The unit of the rate of energy conversion is the Watt, named after the Scottish steam engine pioneer James Watt, and defined as one Joule per second. Heating engineers customarily express heat emission in Kilowatts (kW, a thousand Watts) per hour, commonly just referred to as kW. (Old-fashioned one-bar electric fires were usually 1kW output.) Other units you may encounter are the British Thermal Unit (3412 btu's = 1kW) and the Kilocalorie (860 Kilocalorie = 1kW).

So, how powerful a heater is needed? It is important with solid fuel fires and stoves to pick a heater of the right size - too large and it will have to spend much of its time turned down, leading to smoke, tar and poor performance. Too small and it will be constantly running at a high rate - which means very frequent refueling and rapid parts failure. Both circumstances will tend to give poor Efficiency. It is wise to pick a fire which is capable of fully heating the room, even if the rest of the building is usually centrally heated, so that it can serve as a reliable back-up should the main heating fail.


To more accurately calculate the size of heater needed, we need to know how much heat is being lost through the surfaces of the room (walls, windows, floor etc) and by draughts of fresh, cold, air from outside. Loses from surfaces are commonly expressed as 'U-Values'. High U-values mean bad insulation and lots of heat lost, low U-values mean less heat lost. The calculation is a simple one: the area of surface (in square metres) multiplied by the U-value, multiplied by the difference in temperature between the inside and outside, gives the loss of heat in Watts (per hour).

To find the heat requirement of a single room, fill in the white boxes in the online spreadsheet below in metres and ºC and choose the typical U-values from the drop-down lists. A sample room with three outside walls has already been entered.
To quickly find the heat requirement of a whole building, treat it as a single room.
For L-shaped rooms - calculate as 3 separate rooms

(Download as a spreadsheet:uvaluespreadsheet.xls)

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