:''For energy efficiency in relation to economics, see [[energy conservation]]''
In [[physics]] and [[engineering]], including [[mechanical engineering|mechanical]] and [[electrical engineering|electrical]] engineering, '''energy efficiency''' is a [[dimensionless]] number, with a value between [[0 (number)|0]] and [[1 (number)|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 [[mechanical work|work]] done by the process (in [[joule]]s), and "energy" is the quantity of [[energy]] (again, in joules) used to run the process.
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.
