This article summarizes the experience gained from the study, the siting, the design and the dimensioning of hybrid power plants in autonomous insular power systems in Greece. Wind parks and photovoltaic stations are employed as the RES units, while Pumped Storage Systems (PSS), electrochemical lead acid batteries and electrolysis units for hydrogen production are alternatively investigated as storage units. The examined hybrid power plants operation aims either at the maximization of the RES penetration or at the power demand peak shaving. The hybrid power plants’ dimensioning is optimized using as a criterion either the RES annual penetration maximization or the optimization of the required investments economic indices. The work is integrated with an economic feasibility study for each investigated hybrid power plant. The hybrid power plants presented in this article refer to insular autonomous systems of very small size (Agios Efstratios, Castellorizo), small size (Karpathos–Kasos, Astypalaia), medium size (Rhodes, Samos) and large size (Crete). The article designates how the size of the insular system affects the hybrid power plant’s synthesis and operating algorithm. The investigated hybrid power plants are proved to be technically and economically feasible, regardless of the size of the hosting insular system. The sensitivity analysis of the required investments’ economic indices indicated that the produced electricity selling prices can be lower than the specific electricity production costs in the existing systems. In small and very small power systems, the economic feasibility of the introduced hybrid power plants is achieved with annual RES penetration higher than 90% of the annual electricity consumption.