Technically, the word **efficiency** relates to the ratio of the amount of work one gets out of a machine to the amount of power input. In heat pumping applications, this term is rarely used because it is possible to remove more heat than the amount of power input it takes to move that heat. For a TEC, it is standard to use the term “coefficient of performance” rather than “efficiency.” The **coefficient of performance (COP)** is the amount of heat pumped divided by the amount of supplied electrical power.

The COP depends on the heat load, input power, and the required temperature differential. Typically, the COP is between 0.3 and 0.7 for single-stage applications. However, COPs greater than 1.0 can be achieved especially when the module is pumping against a positive temperature difference (that is, when the module is removing heat from an object that is warmer than the ambient). The figure below shows a normalized graph of COP versus I/I_{max} (the ratio of input current to the module’s I_{max} specification). Each line corresponds with a constant DT/DT_{max} (the ratio of the required temperature difference to the module’s DT_{max} specification).