In the time when renewable sources of energy are becoming more and more available from the economic point of view, it is necessary to consider their impact on existing electro-energy system. This consideration was also the topic of this thesis. Especially this is interesting if you took some island as your main topic to do research. Islands occasionally works as a isolated part of the country and because of this, it is very interesting to do simulation and models of behavior of their electro-energy system. Main reason why the island of Mljet was choosed, is his location and because of that his potential to produce his own energy from renewable sources, especially solar and wind. Also in 2014, it was made a study which reveal this potential and this was a ground floor to create a Action plan for sustainable and efficient consumption of energy for Municipality of Mljet, which was named SEAP Mljet. In the first part of this thesis we will show some basic information about the island, his inhabitants and their way of consuming energy today. Then we will shortly explain software program EnergyPLAN, which will be used for doing simulations and analysis of electro-energy system. Then we will bring some new ideas how to produce even more energy from renewable sources and then in the end we will show the results of our work. The main focus will be in showing: how much is consumption of electrical energy, overall consumption of energy, potentials for producing energy from renewable sources, how much CO2 emissions ARE emitted nowadays, how much are going to be in future and in the end some economics scenarios of this consumptions and potential investments