Question: I have been searching for a definition/explanation of BTU Input and BTU Output and how they relate. Also, how does the "Efficiency" rating relate to these numbers. I have seen some pellet stove manufacturers quote BTU Input and others quote BTU Output. It is a little confusing when comparing stoves capabilities. Answer: OK, that's easy.... Wood or Pellets contain about 8000 BTU per pound...this mean if you measured this in a laboratory with perfect combustion and no heat going up the chimney. In a stove, you have two types of efficiency - Combustion and Heat Transfer. Combustion efficiency is how complete the fuel burns....wood burns very completely in an open fireplace, but you don't get heat because of the poor Heat Transfer efficiency...i.e. everything goes up the chimney. A Pellet stove might burn the Pellets at 95% combustion efficiency and the transfer the heat to the room at 70% efficiency. The total efficiency of the product is one times the other....or 66.5% BTU input x total efficiency = BTU Output. So let's say this pellet stove had a maximum burn rate of 32,000 BTU per hour (4 pounds of Pellets per hour)...it would be using 32,000 BTU of fuel but only delivering 66.5% of that (21,000) BTU OUTPUT to the home.