This paper presents the general proof for a new design principle : the principle of equipartition of forces. The principle has been derived for coupled transports of heat, mass, and charge using irreversible thermodynamics combined with Cauchy-Lagrange optimization procedures. The principle says that the best trade-off between energy dissipation and transfer area is achieved when the thermodynamic driving forces are uniformly distributed over the transfer area. A new strategy for the design of energy optimal transfer processes follows. Practical problems in applications of the principle are discussed.