Wind power generation has increased significantly in the recent decade due to its low-cost and sustainable nature. Determining the rated wind speed (RWS) of turbine is the main factor for generation of electrical energy using wind generators. Till now, no relationship has been reported for determining capacity value using the power curve of the wind turbine as well as the annual energy production (AEP). The purpose of this paper is to determine a proper RWS in variable-speed wind turbines assuming real power factor for maximizing annual energy production and capacity value of the turbine. Aerodynamic power factor and practical efficiency could be extracted from the experimental power curve of the wind turbine. In this study, the annual energy generation of wind is calculated based on integral of nominal wind speed and using a Weibull function, real aerodynamic power factor as well as the practical efficiency of the variable-speed wind turbine. The nominal speed and annual energy production are calculated and assessed for three sites in Poland based on the various nominal wind speed and parameters of the Weibull distribution (shape factor, scaling factor and position factor). Considering the results, proper values for the rated speed of the wind turbine to obtain the maximum annual utilization is significantly different from the rated power of the wind turbine. Thus, if the proposed approach in this paper is not used, the operating capacity of the turbine might be 10% less than the real value. Consequently, it is shown that selecting wind turbines with the proper nominal speed significantly increase energy generation.