The doctoral thesis analyses the issue of the energy system with a focus on the electric power system and the electricity market in the Republic of Croatia, looking at the possibilities of economic cost effectiveness of electricity production from renewable energy sources in order to achieve energy independence and greater sustainability. The purpose of the research is to provide an insight into the state of the electric power system and the potential of renewable energy sources in sustainable electricity production. The main goal of the research relates to possible projections of the economic cost effectiveness and competitiveness of renewable energy sources on the Croatian electricity market. Renewable energy sources represent an important component of modern power systems directed at green transition and sustainable development of the power system in the Republic of Croatia. The optimal model for the introduction of renewable energy sources in the Republic of Croatia was simulated through the PLEXOS model, by setting the scenario of economic cost effectiveness and competitiveness of the model with six power plants (three wind power plants and three solar power plants), four scenarios with prices formed on CROPEX DA (day ahead market), peak load in 2019, and with a forecast for the period from 2022 to 2047. The results of the research demonstrate the cost effectiveness of wind power plants and solar power plants in the scenario "with an increase in the base price by 6 % per year" already in the first year of putting the power plant into operation. The research results also demonstrate that the production of electricity from wind power plants and solar power plants with an installed capacity of 10 MW is not economically cost effective without state subsidies at prices that are formed on the market (except for prices in extreme conditions such as the energy crisis) and with high initial investment costs for the construction of renewable energy sources power plants (solar energy and wind energy). With the minimal predicted price growth in the simulation model (6 % per year compared to the prices from 2019 formed on CROPEX DA), faster construction of new power plants on renewable energy sources in the territory of the Republic of Croatia can be expected. The scientific contribution of the work is manifested in the acceptance of the set scientific hypotheses, which proved that the introduction of renewable energy sources affects the sustainable development of the electric power system and increases the electric power independence of the Republic of Croatia.