The criterion dependencies of the Nusselt number on determining the similarity numbers for two types of electrothermoconvection (in ideal and poor conducting liquid dielectrics) for two types of electro mechanical convection (in emulsions and suspensions) and for three types of electroisothermal convection (electric wind, electrization at generalized unipolar convection, and at a diffusion model) are presented. The main peculiarities of the heat transfer allowing us to calculate the heat transfer process, as well as to under stand the mechanisms and regularities of the EHD phenomena, are revealed.