The problems related to the transformation of electrical energy in mechanical energy and vice versa, involving heat energy (i.e. using thermo-electrohydrodynamic (TEHD) methods) are considered. It is shown that these transformations depend on the type of the dielectric liquid, which can exhibit ideal dielectric properties or weak conductivity. The respective criteria for these cases are given. In an ideal case the mechanical energy can be obtained using only electrical and thermal energy (TEHD pump). In a weakly conducting dielectric a principle possibility exists to generate the convective currents, i.e. to generate the electric energy, using the heat energy and partially the electric energy. The formulas for calculation of the liquid consumption in TEHD pumps and electroconvective currents for TEHD generators are obtained.