Several Mediterranean islands are not connected to the main electricity grid yet meaning that independent local grids cater for their power requirements, mainly using imported fossil fuels whose resources scarcity, especially during the high-demand summer months, is a significant challenge. Seasonal variability in power demand, fuel price volatility, as well as a growing tourism development, impose some additional complexity. The present study analyses specific island grids of the Aegean Sea. Selected islands are analysed as case studies over a 20-year period (2016–2036). The developed energy modelling exercise uses a Harmony Search Algorithm in its core. It investigates whether the deployment of renewable energy sources could lead to a sustainable system, decrease the current burden from the government budget, and support local resources. In doing so we examine whether an increased share of RES, battery storage, and their combination could reduce islands’ reliance on imports and potentially eliminate the current heavily subsidised status quo. The results show that a 30% increase of the consumption will require increases in installed power capacities by 40–70%, in the best case scenario. The latter involves significantly lower per-kWh cost, between 17% and 36% of the current values, due to the large share of RES.