When an electrical generator incurs losses, its efficiency drops from 100 percent. The efficiency of a generator is determined by the power of the load circuit and the total watts produced by the generator. It is expressed as a percentage since you are dividing units of power by units of power. For most commercial electrical generators, this ratio can be upwards of 95 percent. The losses that are incurred typically arise from the transformer, the copper windings, magnetizing losses in the core and the rotational friction of the generator.

Determine the amount of fuel consumed in an hour by the generator. You can use a diesel fuel consumption chart to calculate how much fuel is used in an hour by generators of various sizes.

Determine the total electrical power load consumed during an hour. This can be done by connecting a kilowatt-hour meter to the output of the generator, or by referencing the electrical output label on the generator.

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Convert the amount of fuel used to British Thermal Units, or BTUs. The Oak Ridge National Laboratory has a chart that has the number of BTUs in various types of fossil fuels.

Convert the fuel's BTU value to kilowatt-hours by using the conversion factor of 1 kWh = 3413 BTU.

Divide the output of the generator in kWh by the input value of the fuel used in kWh. Multiply this figure by 100 to express it as a percent.