http://www.woodheat.org/planning/buyright.htm The EPA does not require stove efficiencies to be reported but for obscure bureaucratic reasons assigns default figures of 63 percent for non-cats and 72 percent for cats (and 78 for pellet stoves). Some manufacturers have paid for independent efficiency tests and show the resulting figures on their promotional brochures. But because efficiency test methods have not been standardized and regulated, you can’t be sure the figures are based on the same tests and calculations. On the other hand it appears that all EPA certified stoves are over 60 percent efficient and some can deliver around 80 percent of the fuel’s potential heat to the house. This is far better than the low-tech uncertified options, many of which are in the 50 percent range and lower. An overall efficiency higher than 80 percent is not desirable because the resulting low exhaust temperature means weak draft and the risk of water vapor condensation which damages the chimney.