differential between the cold and hot surfaces therefore in determining the total heat load which a thermoelectric heat pump must transport not only the active heat source of the thermal print head elements must be considered but also the heat leakage associated with a specific mechanical configuration as previously noted a total active heat load q c of 16 7 watts for the illustrated embodiment is expected in addition a heat leakage of approximately 3 3 watts is estimated producing a total heat load q ch of 20 watts using the previously assumed temperature differential of 20 degrees c it is now required to determine the thermoelectric heat pump 39 s operating current and voltage the number of thermoelectric heat pumps needed and the amount of heat rejected q h which is the arithmetic sum of the transported heat load q ch and the input electrical power dissipated in the heat pump fig4 illustrates a typical performance chart for a commercially available thermoelectric heat pump this chart shows the relationship between the heat absorbed at the cold side q c versus operating current the chart also shows the thermoelectric heat pump 39 s coefficient of performance cop versus operating current the running variable is the difference in temperature between the hot and cold sides note that cop is defined as the ratio of q ch to electrical power in and.