Energy transition is probably the major challenge of this century. In particular, the shift toward a clean energy mix gets even more problematic when it comes to realities that lack of infrastructures and financing, such as small islands. The aim of this thesis is to improve the current code of the long-run energy model framework "OSeMOSYS", and use it to model future energy scenarios of San Pietro Island, that has been chosen as case study. Energy modelling and planning is a key topic at this early stage of the ecological transition, since it tries to fill the gap between scientists and policy-makers in the fight against climate change.