An analysis is presented of the losses occurring in a kW-class vanadium redox flow battery due to species crossover, shunt currents, hydraulic pressure drops and pumping, in addition to cell overpotentials. The study was developed on a 9 kW/27 kW h test facility that includes a stack consisting of a 40 cells with 600-cm2 cross sectional area and two 550 L electrolyte tanks. The analysis was carried out combining experimental data and model results. The losses were evaluated as functions of the solution flow rates at stack currents of 30 A, 50 A and 70 A. Charge-discharge cycles performed at a constant flow factor equal to 8 and thus at variable flow rate were analyzed. Results show that shunt currents have a tenfold to twentyfold higher effect than hydraulic pressure drop inside the stack. As regards cell membranes, it has been found that, in terms of efficiency, the ion conductivity (i.e. cell overpotentials) counts more than species diffusivity (i.e. crossover). Moreover, it was found that pumps and their motors can affect significantly the efficiency of a kW-class vanadium redox flow battery system.