Heating output is specified in kilowatts, kW, (metric units) or BTUs (Imperial units) and represents the rate at which the system can deliver heat energy.

Sizing of heating systems should be done by a qualified heating engineer. It is dependent on many factors including levels of insulation and draught proofing of your home, lowest outside temperature of your locality and patterns of use.

However the following ‘rule of thumb’ can be useful for making initial sizing estimates for central heating boilers:
Boiler size (in kW) = volume to be heated (in cubic metres) divided by 34 (for a reasonably well insulated house).

It is important to bear in mind that all biomass boilers burn most cleanly and efficiently when working at their maximum output. Therefore it is best not to over-specify but to choose a biomass boiler which is sized to meet your average heating requirements with additional heating sources to provide extra heat on the coldest days.