Hi Kent,
This is a interesting question.
I found a
reference for generic charcoal containing 9,000 BTUs per pound. I found a
reference for Kingsford that indicated it contained 9,700 BTUs per pound. I don't know the accuracy of these values, but guess that they are in the ballpark.
My thought is that, all else being equal, the BTUs expended to cook an item is highly dependent upon the efficiency of the particular cooker and the cooking methods used for each type of cooker--propane fired and charcoal fired.
Why not track the pounds of each, propane and charcoal, used for similar cooks. Multiply the pounds of each fuel used by the cost per pound of the fuel. This method should give you a reasonable approximation of the cost of using charcoal versus the cost of using propane when cooking similar foods under, of course, similar conditions. Tracking multiple cooks under this scenario would likely give improved estimates of costs.
My
guess is, that at nominal prices for both propane and for charcoal, charcoal would be more expensive to use in most smoking or grilling use. Despite any cost difference, one should also consider all the other advantages and disadvantages of using each fuel source.
###