The standard methodology involves measuring your room and dividing this by a certain number. Length x width x height in metres divided by <14>. So 5m x5m x 2.4m = 60 cubic metres of air in the room. 60 divided by 14 = 4.28. So the room requires approx. a 4.28 kW product to heat it effectively. Note that dividing by 14 is for an average insulated property with simple double glazing. An old drafty property divide by 10 and a well insulated new build by 25.
So now to stoves. Sticking to basics the amount of heat a stove gives out is determined by how much wood is burning at any one time.
So it follows that the physical size of the firebox (the inside volume of the stove) determines the maximum heat output of the stove (the bigger the inside of the stove the more wood can be burned so the more heat can be emitted).
Yet we see stoves with tiny fireboxes advertised as nominal 5kW and stoves with HUGE fireboxes also advertised at nominal 5kW. Don't believe the figures is the simple answer. The tests only prove that the stove is capable of reaching that output with a reasonable loading of logs. The real figure that would be useful is the maximum output and manufacturers rarely provide this. Some 5kW stoves are capable of 8kW just by adding an extra log or two. Or even 10kW if you really stack it. But, taking things to the extreme, one runs the risk of it all ending up on the hearth when you open the door! So some 5kW stoves really will just be capable of 5kW whilst other can easily and sensibly cope with a couple of extra logs.
Why might a manufacturer want a stove to be advertised as 5kW when one can add 8kW worth of logs? Because stoves 5KW or less often do not have to have an air vent in the room (which is a selling point). Also the efficiency rating is measured at the kW point chosen by the manufacturer. As you can see a manufacturer can play around with the figures – choosing between efficiency/kW output as they desire.
My advice: If you are in a wood stove retail outlet and looking at stoves then take the manufacturer’s stated kW output as a guide only. COMPARE THE SIZE OF THE FIREBOX WITH OTHER STOVES TAKING INTO ACCOUNT HOW MANY LOGS ONE CAN SENSIBLY ADD TO THE STOVE.
Hence why I have added “Firebox size” to all of my reviews (see icons & terminology).