Electricity consumption increases continuously because of several reasons such as population growth. Due to consumption growth of electricity it is necessary to upgrade generation, transmission and distribution equipments. In distribution level, transformers of sub-transmission substations should be upgraded to overcome load growth. In this paper, it is recommended to use wind generators and storage devices instead of transformer upgrading of substations. This is significant because of environment pollution and investment deferral of sub-transmission substations. Hence, this paper suggests a new method to determine the optimal capacity of wind generators and energy storage system (ESS) for investment deferral of sub-transmission substations. In this method reliability and economic aspects are considered along with time varying loads. Furthermore, to peruse the uncertainty of wind generation an innovative point estimate method (PEM) has been fulfilled. The main goal of the presented method is to minimize the investment cost of the ESS units and wind generators, purchased power from upstream network and also reliability maximization. The objective function is mathematically formulated as a mixed-integer nonlinear programming problem and subsequently solved by genetic algorithm (GA). The proposed method is successfully applied to a study case and obtained results show the efficiency and applicability of the proposed approach.