In this thesis, based on measurements of the water flow at the water measurement station Nadvode, 7245 on the Zrmanja river in Croatia, conducted over the course of several years, the energy hydropotential of the potential Žegar hydropower plant was calculated. The Zrmanja river is not yet utilised for harvesting energy and as such offers possibilities for harvesting energy. Current energy trends emphasise to so-called renewable sources of energy. People have become aware of the fact that the utilisation of fossil fuels is limited and that at the same time their consumption causes great harm to the environment. On the other hand, hydropower plants, as a source of energy, emit almost no emissions in the course of their operation and do not consume other energy products. Therefore, they are classified as renewable sources of energy. For this purpose, the Croatian Rules on the Support for Energy Produced by Renewable Sources of Energy have been analysed. Based on geodetic data, a model was made of the terrain in the surrounding area of the Žegar hydropower plant as well as certain main technical characteristics of the power plant, which may in any way affect the calculation of the hydro-energetic potential of the Žegar hydropower plant. Data about the medium daily flows during the period of measurements was processed, evaluated and later considered in the calculation of the hydro-energetic potential. The hydro-energetic potential of the location was calculated for two types of hydropower plants, namely for the derivational hydropower plant and the accumulative hydropower plant. Both potentials have been compared with each other. Later, both types of hydropower plants were cost-assessed as well. Finally, based on the previously calculated hydro-energetic potential, an economic analysis of the feasibility of possible investments in both types of hydropower plant was conducted, this was achieved based on annual revenues, expenditures and the ability to pay back a possible loan for the construction of the hydropower plant.