In an earlier post I asked the question about how the EPA tested the OWBs for a efficiency rating. I really was not trying to get to the method involved, I was basically getting at how could they determine the efficiency(more of a philosophical question) They can do a test and say OWB "A" gets more BTUs then OWB "B" from the same amount of wood but if I only spend $200 for gas and I save $2000 by not buying propane, I would think that the EPA formula would need to be: total BTUs from wood burned/ total BTUs available from $200 of unleaded gas that I used to get the wood. Because the wood I burned grew without any help from non renewable sources.
It is kind of like saying one solar panel is 12% efficient and another is 14% efficient, because all the energy collected from the sun is free so if the input energy is more or less free who cares?