Starting with a simple transport equation for entropy and electrical charge, the power conversion and its efficiency are deduced for a single thermoelectric material apart from a device. The material’s performance is discussed on the material’s voltage-electrical current curve, which is presented in a generalized manner by relating it to the electrically open-circuited voltage and the electrically closed-circuited electrical current. Particular working points on the material’s voltage-electrical current curve are deduced. It is shown that optimizing a thermoelectric material in generator mode for maximum electrical power output - based on the power factor - or optimizing it for maximum efficiency - based on the figure-of-merit - are different tasks. A similar behavior of a deviation between maximum efficiency and maximum electrical power output can be found for the optimization of thermoelectric generators. Here, the vast number of various material classes makes it difficult to maintain an overview of the best candidates for a specific task. For this purpose, Ioffe plots are revitalized, which facilitate the clear comparison of thermoelectric properties of different materials to either access high electrical power output or high conversion efficiency.