Book excerpt: In this book, Chapter One proposes new tools for energy planning under sustainable development paradigm through models, procedures and methodologies. Chapter Two focuses on Home Energy Planning Systems applied to the residential sector. In Chapter Three, the possibilities to use geothermal technologies in Spain and whether they can reach generation objectives is examined. Chapter Four focuses on the impact that the grid regulatory framework, in force in several islands has had both from the technical and economical point of view, regarding their renewable energy development.