A simulation model of a vanadium redox flow battery (VRFB) system based on measurements with a kilowatt scale real life VRFB unit was developed. Various hourly charging and discharging cycles were performed for states of charge (SOC) of 75%, 60%, 40% and 20% at different power values ranging from 2 kW to 10 kW. The dependence of the overall system efficiency in a VRFB unit on the SOC was determined by considering the energy losses at stacks during the electrochemical conversion and by the auxiliary power consumption of the hydraulic circuits as well as the power conversion systems. Using the model, optimal number of modules for certain power levels during charging and discharging operations were estimated for megawatt scale operations. (C) 2012 Elsevier Ltd. All rights reserved.