Thermoelectric coolers are characterized by their heat pumping capacity and maximum temperature difference, or Delta-T. However, most do not realize that these two performance metrics can never be achieved at the same time. Maximum heat pumping, or Q-max, can only be achieved when the temperature difference across the device is 0. Likewise, the maximum temperature difference, or Delta-T max, can only be achieved when the heat pumping is 0 Watts. The use cases for thermoelectric coolers will be somewhere in-between these maximum points and depend greatly on the system as a whole, including the cold- and hot-side heat exchangers and ambient conditions.