Numerical modelling of redox flow battery (RFB) systems allows the technical and commercial performance of different designs to be predicted without costly lab, pilot and full-scale testing. A numerical model of a redox flow battery was used in conjunction with a simple cost model incorporating capital and operating costs to predict the technical and commercial performance of a 120MWh/15MW utility-scale polysulphide-bromine (PSB) storage plant for arbitrage applications. Based on 2006 prices, the system was predicted to make a net loss of 0.45pkWh−1 at an optimum current density of 500Am−2 and an energy efficiency of 64%. The system was predicted to become economic for arbitrage (assuming no further costs were incurred) if the rate constants of both electrolytes could be increased to 10−5 ms−1, for example by using a suitable (low cost) electrocatalyst. The economic viability was found to be strongly sensitive to the costs of the electrochemical cells and the electrical energy price differential.