Typically a designer will target values for:

1. the desired Delta T (i.e., the difference in temperatures present at each mounting surface of the TE device), and

2. the thermal load (in Watts). Usually, the required Delta T will have to be 15-20°C greater than the difference between the thermal load and the ambient environment due to losses in thermal interfaces.

Each slanted data line on the lower portion of the graph above, shows all of the combinations for Delta T and heat-pumping capacity at the indicated levels of TE current (in amps DC). In this example, the designer identifies a point on the 4.5 amp data line which will yield a Delta T of 45°C at a thermal load of 17.5 watts. The TE voltage is determined by finding the corresponding Delta T point on the 4.5 amp data line in the top portion of the graph; here we see that it will take 11.8 volts DC to power the TE device at this operating point.