To optimize the design of a thermoelectric heat pump for minimum input power, it is necessary to make a good estimate of the heat load. Multistage thermoelectric heat pumps are required to cool temperature sensitive components to 233°K (-40°C) or colder. In these cases the relationship of Coefficient of Performance, COP, vs Cold Side Temperature, TC indicates that any small error in the heat load calculation is reflected as a large error in input power. The relationship of heat load and input power relative to cold side temperature is discussed in this paper. Graphical presentations are used to estimate the heat loads for these examples and to show the effect on input power. Typical examples are as follows: 1. Cooling of a multielement infrared detector array to 193°K (-80°C), 2. Cooling of a single element silicon laser sensor to a temperature of 163°K (-110°C), 3. Cooling a self scanned array DIP to 233°K (-40°C) in Nitrogen and 4, Cooling a self scanned array chip to 233°K (-40°C) in Zenon. Each example has specific problems in evaluating heat load and input power requirements.