The objective of this article is to investigate the technically and economically optimum electricity storage technologies for small, insular, autonomous electrical grids, integrated with Renewable Energy Sources (RES) power plants. Three autonomous Greek islands are investigated as case studies: Symi, Astypalaia and Kastelorizo, with annual peak demand at 4.0 MW, 2.2 MW and 0.9 MW respectively. All three islands exhibit excellent wind and solar potential, with ideal sites for the installation of seawater Pumped Hydro Storage (PHS). Two different approaches are investigated, regarding the electricity storage plants: PHS systems (for the two largest islands) and electrochemical storage, alternatively realized with lead acid or lithium-ion batteries. Wind parks and photovoltaic stations are considered as the potential RES units. Relevant operation algorithms are introduced. The dimensioning of the examined plants is optimized with a common target: the achievement of RES annual penetration percentage higher than 70%, ensuring the investments’ economic feasibility, with electricity selling prices lower than the existing specific production cost. Given the favorable land morphology for PHS installations, it is shown that wind-PHS still remains a competitive alternative for Symi and Astypalaia, despite their relatively small size, while for Kastelorizo, a wind-photovoltaic-batteries features as the optimum option. 100% annual RES penetration can be achieved only with the PHS support. With electrochemical storage systems, the RES annual penetration can be between 80 and 90%. The economic feasibility is ensured with electricity selling prices between 200 and 350 €/kWh. The investments’ payback periods are estimated between 6 and 10 years.