Cost-optimal electricity system configurations with increasing renewable energy penetration were determined in this article for six islands of different geographies, sizes and contexts, utilizing photovoltaic energy, wind energy, pumped hydro storage and battery storage. The results of the optimizations showed strong reasoning for islands to invest in renewable energy technologies (particularly wind energy), as compared to conventional power generation. Levelized cost of systems for electricity generation decrease considerably with increasing renewable energy penetrations, to an optimal point in the range of 40-80% penetration. Furthermore, renewable electricity integration in the order of 60-90% could still be achieved with no added cost from the initial situation. Cost increases after these optimal points are attributed to the growing inclusion of storage, required to meet the higher renewable energy shares. However, with battery costs forecast to fall in the coming years, and a cost reduction of 50-70% already causing lithium-ion batteries to overtake pumped hydro as a cost-favorable storage option in this model, there is a real case for islands to begin their transition in a staged process; first installing wind and PV generation, and then - as storage costs decrease and their renewable energy capacities increase investing in storage options.