How is wood-burning stove efficiency measured?

Wherever possible, we give you efficiency information on the stoves that we sell. But where do these figures come from and what do they mean?

You will find efficiency data on most our product pages. This is not to be confused with the heat output figure, which will be given in kilowatts, such as 8kW.

The efficiency figure, which will be given as a percentage, is designed to tell you how much of the fuel you burn on any given appliance will actually be used on heating your room, and how much will be lost during the process. So, a stove with efficiency of 80% is losing 20% of the heat the fuel could potentially give off straight up the chimney.

By way of context, an open fireplace has approximately 30% efficiency, meaning that a huge 70% of the energy is being lost.

The measuring process

Stoves are usually tested by independent energy consultancies in accordance with criteria set by British and European standards bodies.

However, the stove manufacturer is given considerable leeway as to the circumstances in which their appliance will be tested.

For instance, they can choose:

  • the heat output at which the stove will be tested
  • the size of the fuel being burnt during the test
  • how often the stove is refuelled, as long as its no less frequently than every 45 minutes

The consultant then measures the temperature and carbon content of the flue gases. Since care is taken to know precisely how much fuel is being burnt, these measurement allows the tester to establish how much heat and how much fuel has been wasted. The stove efficiency is calculated from those figures.

The difficulties of stove efficiency measurements

While it is useful to know the potential efficiencies of a stove you’re considering buying, the measurements are not without their drawbacks.

As you’ve probably considered by now, if each manufacturer is setting individual testing environments for their stoves then a like-for-like comparison between different brands becomes almost worthless.

Another problem is that it is easy for a manufacturer to adjust the refuelling period, heat output and air supply (by closing vents) to achieve the efficiency level they want.

In short, you can’t necessarily use the stove efficiency figures as a reliable guide as to which stove will be most efficient when you come to use it at the temperature you want, using the fuel you want to burn and refuelling as regularly as you want to.

Browse our full range of wood-burning and multi-fuel stoves.

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