Efficiency is a measure of how much useful [[Energy|energy]] can be extracted from a system for a given input energy. Efficiency is commonly expressed as a percentage. We must keep in mind what effect you desire out of a system. For instance, the useful energy out of a [[Heat Engine|heat engine]] is [[Work|work]], while the input energy is [[Heat|heat]]. When using heat to create work we measure efficiency as thermal efficiency, $\eta$. $η=\dfrac{\text{Benefit}}{\text{Cost}}​=\dfrac{\dot{W}_{net}}{\dot{Q}_{in}}​​$