Some special features like having no moving parts and long lifetime and being highly reliable make thermoelectric generators (TEGs) a proper choice to convert solar energy into electricity. In this work, performance of Bi2Te3 and Zn4Sb3 solar thermoelectric generators (STEGs) are studied under transient condition using both experimental and numerical approaches. Variation of the temperatures of the hot and cold sides of the TEGs, open circuit voltage, short circuit current and maximum power generation to the fluctuation of the solar radiation, simulting semi-cloudy weather, are obtained and discussed. Effect of thermoelectric material properties and geometry of the thermoelectric elements on electrical response and performance of the STEGs is invetigated. Results of the developed numerical model using finite volume method are in good agreement with the experimental data. Performance of the TEG without graphite layer and with the same solar radiation condition is investigated and compared with the TEG with graphite layer. The results show that, applying the graphite layer significantly enhances performance of the STEGs. It is found that using graphite layer enhances the efficiency of the STEG system. Furthermore, for high solar concentrations, Zn4Sb3 based STEG module has better performance compare to the Bi2Te3 based module.