Efficiency compares the output produced by a power system with the total energy input into the system. For example, to figure the efficiency of a self-propelled lawn mower, you compare the energy input (the electricity or fuel used by the mower) with the work output (the speed and force of the whirling blade, the amount of ground covered, uphill travel, etc.)

How efficient a machine is can be expressed as a number obtained from this general formula:

Useful Work/Energy Input = Efficiency Rating

The ideal power system would be a device that converted a readily available form of energy directly into useful work with no loss of energy, i.e., with 100 percent efficiency.