This paper presents a methodology for determining the optimal size of a Vanadium Redox Battery (VRB) which supplies secondary frequency control in electrical power systems. The objective is to achieve the optimal investment for installing a VRB while taking into account its impact on the power system. Through stochastic optimization, the proposed methodology allows computing the variable operative costs and VRB investment costs. To achieve this, an interior point algorithm within an exhaustive search is used. In order to analyse the influence of the VRB on frequency quality of the power system, statistical index is proposed such as the new factor used in dimensioning the secondary reserve.