In physics and engineering, including mechanical and electrical engineering, energy efficiency is a dimensionless number, with a value between 0 and 1. The energy efficiency of a process is defined as:

<math> \mathrm{efficiency} = { W \over \mathrm{energy}} <math>

where W is the amount of useful work done by the process (in joules), and "energy" is the quantity of energy (again, in joules) used to run the process.

Energy efficiency can also refer to the use of techniques or practices that minimize the use of energy.

An efficiency of 1 (100%) is impossible - the second law of thermodynamics prohibits it. Theoretically, the mutual annihilation of matter and antimatter would be completely efficient, but the collection of the released energy would not be.