Performance prediction of thermoelectric generators (TEG) is an important work in thermoelectrics and a physical model is quite necessary. Now basic thermoelectric phenomena have been expounded explicitly, modeling a TEG is an accessible work. However, the Thomson heat (which is a second-order effect) is usually neglected in device-level TEG analyses. And the dealing with the output power expression without Thomson heat is improper in some studies. Based on a thermoelectric model which considers basic thermoelectric effects, as well as the thermal resistances between the thermocouple and the heat source, heat sink, reasonable expressions of Thomson coefficient and Seebeck coefficient are proposed. The output power expression without Thomson heat is analyzed and redressed. With and without Thomson heat, the output power and energy efficiency are calculated at different thermal conditions. Some new results distinct from the past ones are presented. At last, in order to testify the physical model, a BiTe-based thermoelectric module is tested and an ANSYS model is built.