This paper presents a methodology to evaluate the optimal capacity and economic viability of a hybrid energy storage system (HESS) supporting the dispatch of a 30MW photovoltaic (PV) power plant. The optimal capacity design is achieved through a comprehensive analysis of the PV power plant performance under numerous HESS capacity scenarios. The analysis has been conducted using a high performance computing cluster which generated a large amount of simulation data based on a PV power profile of one month. The HESS, consisting of a vanadium redox battery and a supercapacitor bank with a power rating ratio between the two energy storage technologies of 5:1, is connected at the point of common coupling to support the PV power plant to comply with the dispatch rules in the Australian national electricity market. A quantitative relation is developed based on surface fitting, which relates the HESS capacity with the dispatch performance of the PV power plant in terms of energy yield and required ancillary services. The paper concludes the optimal capacity of HESS which will provide the maximum profit improvement under the actual market conditions.