When comparing the performance of heat pumps, it is always good to avoid the word "efficiency", which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement per work input. Most vapor-compression heat pumps use electrically powered motors for their work input. However, in many vehicle applications, mechanical energy from an internal combustion engine provides the needed work. According to the US EPA, heat pump can reduce energy consumption up to 70-80% compared with traditional heating systems. Heat pumps in general have a COP of 4.2 to 5.2 which places it behind co-generation with a COP of 9.