Using the WTC3243 for controlling the temperature of an aluminum block, at 1 A with a 12 V power supply, what is the power generated?
At 1 A, the WTC3243 and the WTC32ND have a minimum compliance voltage of |VS-1.2| V out to the TEC from its internal supply. So since your current is 1 A, and the voltage drop across the WTC3243 is 1.2 V, your power dissipation will be 1.2 W, i.e. I X V = P, 1A X 1.2V = 1.2W.
The minimum and typical compliance voltage values for specific load current levels are given on page 8 of the WTC3243 Datasheet so you can also calculate this for other current values.