Isothermal electrohydrodynamic phenomena in three important approximations—the isothermal, hydrostatic, and electrohydrodynamic equilibrium of dissociation–recombination processes of charge transfer in liquids—are considered. For the known mechanisms of isothermal charge formation, the influence of the electrohydrodynamic phenomena on the heat transfer is estimated by means of criterion dependencies obtained on the basis of determination of the scale of the moving force of electroconvection. The results may be used for calculations for electroconvective heat exchangers and for further study of isothermal electrohydrodynamic phenomena.