New concepts of power generation, the emergence of renewable energies as well as the liberalization of the electrical market have recently raised strong intereset in electricity storage technologies. The vanadium redox flow battery (VRB) is among the new storage systems actually developed. A model of VRB storage system is presented in this article based on thermodinamics and electrochemistry. The stack voltage and the different concentrations of vanadium species are determined by the model and depend on the operating state of the system. Finally, a 24h simulation of a system composed of a photovoltaic source, a VRB and a typicaluser is presented. The aim is to improve the specifications of the static converter interfacing the different components of a VRB electricty storage system.