The final energy consumption in buildings in Europe is more than 40% of the total energy consumption and is growing steadily due to the constant increase in living standards. At the same time, building sector has the highest potential for ecological and energy savings and the implementation of renewable energy sources. Croatia itself is rich in renewable energy resources, especially the Dalmatian region with its Mediterranean climate that, according to UNDP, has a great predisposition for a self-sustainable region. In order to begin to rely on on- site renewable resources a series of activities at the national level, and especially at the local level is required. Due to the favourable geographic location, concern for the pupils and the environment, the Elementary school Ostrog wants to become the first energy independent school in Croatia, meaning it produces as much energy on site as what is consumed throughout the course of the year. This thesis focuses on examining the best and most feasible ways to meet the school's need for energy: through combination of energy efficiency measures and implementation of locally available renewable sources. Through a dynamic simulation software two scenarios were applied: the refurbishment of the lighting system and the thermal improvement of the opaque part of the building envelope. The results were compared and evaluated according to the different criteria: electricity and heating demand. The renewable energy potential on site was calculated and explored if it can cover the school's annual need for electrical energy through installed photovoltaic system and heating demand through biomass from olive pomace. Finally, the financial costs for proposed measures were calculated, as well as possibilities for financing them.