The measure of usefulness.
Efficiency is the measure of "usefulness" of an operation, process or machine - and can be expressed on the generic form
μ = Wo / Wi (1)
μ = efficiency
Wo = output from the operation - can be work, power, produced products ...
Wi = input to the operation - can work, power, input products ...
Efficiency can be expressed as a percentage or as a per-unit decimal fraction of 1.
Example - Efficiency of an Electric Motor
The input power to an electric motor is 2000 W. The output power to the shaft is 1800 W. The efficiency of the motor can be calculated as
μ = (1800 W) / (2000 W)
Example - Efficiency of a Wind Mill
The teoretical energy potential for a wind mill is 300 kWh at given conditions. The power produced is 70 kWh. The efficiency of the wind mill can be calculated as
μ = (70 kWh) / (300 kWh)