In this paper, a general theory of the photothermally induced Seebeck voltage in the harmonic regime is presented. Conditions highlighting coupling effects between temperature and electric fields are reported. In the frequency domain, thermoelectric materials are governed by two regimes separated by a cut-off frequency. Below this point, there is no manifestation of coupling on the voltage response. In its vicinity and beyond, coupling appears through an effective Seebeck coefficient and an effective thermal conductivity. From this last quantity, the figure of merit can be, in principle, theoretically determined from the ratio of thermal conductivities obtained respectively at high and low frequencies or from the maximum value of the complex thermal conductivity argument at the cut-off frequency.