The annual fuel utilization efficiency (AFUE; pronounced 'A'-'Few') is a thermal efficiency measure of combustion equipment like furnaces, boilers, and water heaters. The AFUE differs from the true 'thermal efficiency' in that it is not a steady-state, peak measure of conversion efficiency, but instead attempts to represent the actual, season-long, average efficiency of that piece of equipment, including the operating transients.
The method for determining the AFUE for residential furnaces is the subject of ASHRAE Standard 103. A furnace with a thermal efficiency (Î· th ) of 78% may yield an AFUE of only 64% or so, for example, under the Standard's test conditions. When estimating annual or seasonal energy used by combustion devices, the AFUE is the better efficiency measure to use in the calculations. But for an instantaneous fuel consumption rate, the thermal efficiency may be better.
To calculate the efficiency of a fuel cell, you find the percentage therefore % of fuel efficiency=Power out/Power in then multiply the result by 100. Fuel cells are made up of three segments which are the anode, the cathode and the electrolyte. See more details at http://en.wikipedia.org/wiki/Fuel_cell.