Vanadium redox flow batteries (VRB) are large stationary electricity storage systems; their performance is enhanced when they are operating under optimal conditions. The multiphysics model developed in this paper offers a good understanding of the VRB operating principles; this knowledge is important to identify and quantify the source of losses: the internal losses and the auxiliaries losses. The model is divided in two main parts: the electrochemical model that predicts the stack voltage, the variation in vanadium concentrations and the internal losses under any operating conditions and the mechanical model that describes the hydraulic circuit of the electrolyte and determines the pumps power. Then the stack efficiency is determined with the electrochemical model in a series of charge-discharge cycles and compared with experimental data. Finally, a method is proposed to increase the VRB performance by minimizing the power required to flow the electrolytes; in a 19 elements stack, the average gain is more than 1600 W.