The development of technologies and the awakening of environmental awareness has accelerated the growth of electricity production from renewable sources. In the last ten years, most countries in Europe and the rest of the world have shown a significant trend of increasing the installation of photovoltaic power plants. In the Republic of Croatia, which has proven to be a country with great potential, a significant increase in photovoltaic power plants has begun to take place. Photovoltaic power plant "Kozjak", the main topic of this final thesis, is one of the positive examples of the use of solar energy. In this final thesis, the topic of photovoltaic power plants is discussed and their main elements are described. The methodology of calculation of electricity production estimation using PVGIS "online" calculator is presented. The data on the produced electricity of the above-mentioned power plant were also processed and analysed with the estimated data obtained using the PVGIS "online" calculator.