Thermoelectric materials can show high performance of energy conversion. Many models predict this behavior, however in long term operation many problems arise such as degradation due to sublimation. This might mean that long term operation at high temperature needs to be modeled with real numbers representation degradation in order to predict long term efficiency. In the current research a model was developed using both the evaporation rates at the high temperatures and the material degradation at high temperature to model the long term operation of such a device. The device was modeled for 60°C cold side to 600°C hot side operation using PbTe based materials showing the differences between 0 hour degradation to 456 hour degradation. This was investigated both experimentally and theoretically. Surprisingly to the opposite nature of some of the effects only small degradation to the maximal electrical power and efficiency was observed. This shows that degradation is only a moderate effect in such devices.