Recall that the efficiency rating of a system or machine is calculated by comparing the amount of energy input to the amount of energy output (work accomplished).
Finding the difference in efficiency of different fuels is complicated, because the fuel volumes are measured in different ways according to their forms: solid, such as coal; liquid, such as gasoline; or gas, such as natural gas. Conversion is often required, because to be accurate, inputs and outputs must be expressed in the same units, such as Btu's.
Efficiency is usually given as a percentage and can be computed with the following formula:
For example, if you ate only one apple for every two apples you were given, your apple-eating efficiency would be calculated like this: