The unit of the rate of energy conversion is the Watt. Heating engineers customarily express heat emission in Kilowatts (kW, a thousand Watts) per hour, commonly just referred to as kW.

So, how powerful a heater is needed? It is important with 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 refuelling 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.


As a very rough guide it has long been customary to take a figure of 1kw per 12 metre³ of space to be heated. The equation here would suffice as a rough guide

Metric:   Length x Width x Height

A room of 6m x 4m x 2.4m would require a stove of 4.8 kw


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 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). A heat engineer could easily determine this for you.

Use our nifty room heating calculator below to
identify if this stove is adequate for your needs:
Room Measurements in:
Room Length:
Room Width:
Room Height:
  Click here to Calculate Size
Heat Output Required (kw):

Note: The Heat Output Calculator is based on the dimensions of the room and the insulation level this form estimates the heat loss and suggests a stove with a heat output in excess of the heat loss to provide a quick heat up time. If your room has a complex shape, or has more than 2 external walls, or has a large amount of glazed area, then we would strongly advise getting a professional heat loss evaluation carried out. This is for a non boiler stove only.

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