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