The focus of this book is the application of Linguistic Weighted Average (LWA), where the weights are always words modeled as interval type 2 fuzzy sets (IT2FS) for reliability analysis of interconnected power systems. Reliability analysis has been considered as an important step in any system design process. A reliable electrical Power system means a System which has sufficient power to feed the load demand during a certain period. With the interconnection of Photovoltaic (PV) and Wind Energy System (WES) as a Hybrid Electrical Power System (HEPS)into the Utility Grid (UG), the fluctuating nature of the energy produced by these systems has a different effect on the overall system reliability than that of the fluctuating natures of energy produced by UG.The complete study to determine the impact of interconnected PV/WES/HEPS into UG from reliability point of view is presented. A fuzzy logic technique using Type 2 fuzzy sets has also been presented to calculate and assess the reliability status for each HEPS configuration under study.