Isolated islands energy systems’ strong dependence on imported fossil fuels and renewable resource potential make them ideal test-beds for Renewable Energy Source (RES) penetration energy planning. La Gomera, part of the Canary Islands, stands out as a particularly strategic case study due to its ongoing transition away from fossil fuels and its strong renewable energy potential. However, the intermittency of renewable sources and the variability of demand and market prices introduce significant uncertainty into planning decisions. This thesis provides a replicable framework for the optimal sizing of a renewable portfolio, composed of onshore and offshore wind, and solar photovoltaic, and of a Battery Energy Storage System (BESS) for the island of La Gomera.