There are many misconceptions about efficiency ratings that the marketing gurus love to throw around to confuse the consumer.The pellet stove efficiency rating published by their manufacturers is a combined rating which includes combustion efficiency, electrical efficiency and heat transfer efficiency. Pellet stoves mostly burn at over 98% combustion efficiency, and the electrical efficiency is about 99%. Assume, for the sake of argument, that the heat transfer efficiency of a pellet stove is 60%. If you add up 98%, 99% and say 60%, you get 257. Divide that figure by three to get the overall efficiency and it comes out at 85.7%.
78% is the EPA's assumed default efficiency for a pellet stove's overall performance. Its clear then, that the heat transfer efficiency of pellet stoves is often less than 60%. The published Btu rating of a pellet stove relates to its combustion efficiency. This is a measure of the heat produced from burning fuel. It does not directly relate to heat available to the home as some of it disappears up the flue.
One pound of hardwood pellets will produce around 8,200 BTU’s. Softwood pellets slightly more. As combustion efficiency is so good in pellet stoves, this figure is very close to the actual heat 'input' of the stove. So take a stove with a published rating of 40,000 BTU/hr. Divide 40,000 by 8,200 and you see that at that output, the stove is burning 4.8 lbs. of pellets an hour. This 'heat input rating' is the main figure we have for assessing the capability of a pellet stove.
As well as assuming an efficiency of 78% for pellet stoves, the EPA also stipulates that they produce particulate emissions of less than 2.5 grams per hour to be approved. Tests for this must be carried out at an independent testing laboratory such as Omni. To be exempt, a pellet stove must have an air to fuel combustion ratio of more than 35:1.
From the above heat transfer efficiency figures, it's clear that there must be a wide range of heat exchanger efficiencies in pellet stoves. To be effective a heat exchanger must have a large surface area, and the stove must direct hot air evenly over that area. Bottom line? Pellet stove efficiency is only as good as the efficiency of its heat exchange system