The optimal design of Hybrid Photo Voltaic–Wind Turbine (PV–WT) grid connected power generation systems (PGSs) is not an easy or straightforward task, as the best configuration depends on many different uncertain socio-economic and environment variables, design parameters, technical constraints and government and policy maker interests. In this context, this work deals with the optimal design of grid connected PV–WT PGSs, achieved by using a suitable procedure based on a Multi Criteria Decision Analysis (MCDA) optimization approach. The method takes into consideration all the aforementioned issues, working toward optimizing amount of reduced emissions, total costs, and social acceptability, and providing a clear overview of the most suitable PV–WT design configurations. The proposed approach has been applied to different case studies and uncertainty analysis related to solar radiation, wind speed and load demand profiles variations has been carried out using a probabilistic approach; in addition, different criteria weights have been adopted for sensitivity analysis, where the importance of each design parameter has been assumed to be uncertain, and its effect on the final optimal design is investigated. Results confirm the effectiveness and flexibility of the proposed approach, which could be assumed as a powerful decision making tool in designing hybrid renewable energy grid connected PGSs.