The efficiency of thermoelectric devices in converting heat into the electricity is defined as the ratio of ``energy provided to the external load'' to ``heat energy absorbed at the hot junction''. The maximum efficiency of the power generator is achieved when the external load is matched with the device resistance [3]:
(1.8) 
(1.9) 
(1.10) 
The efficiency of a thermoelectric power generator, as any other heat engine, is less than the Carnot engine efficiency:
(1.11) 
Indeed, the Carnot engine has the most efficient cycle for converting a given amount of thermal energy into work. In addition to the temperature difference, is also related to the average of , which is related to the material properties of the type and type materials used in the thermoelectric element. Figure 1.5 shows the maximum efficiency of the thermoelectric generator as a function of for and different . The efficiency increases with and . For , the maximum efficiency is about . The efficiency for high increases to and for and , respectively. The efficiency of Carnot cycle is plotted as well for comparison. We also note that the efficiency of some other common energy convertors is as follows: for hydroelectric technologies, for fuel cells, for wind turbines, and for tidal turbines [1]. As a result, an average figure of merit higher than 3 is required, in order to compete with the rest of the commercial generators that are already in largescale use in the market [4].

The thermoelectric device figure of merit and the efficiency are related to the material properties of the type and type semiconductors as well as the electrical and thermal contact resistances. For simplicity, the thermoelectric material figure of merit:
(1.13) 