This paper deals with the description of the working principles of reversible hydroelectric power plants and their purpose as electric energy storage facilities. Reviewing the Croatian Electric Energy Power System has given an insight in the importance of hydroelectric power plants regarding the total electric energy production in Croatia. As well as that, a general classification of hydroelectric power plants was made. A simple mathematical model on the example of the reversible hydroelectric power plant Velebit has been carried out which gave some of the fundamental values of every hydroelectric power plant and also described the Velebit facility in more detail. Furthermore, the RHE Velebit was put in a wider perspective by comparing it with some of the largest and most advanced similar facilities in the world. In the end, a theoretic solution for storing the electric energy produced in wind farms has been introduced. It was described more in detail on the example of a project that aims for turning The Hoover Dam hydroelectric power plant and its accumulation lake in a gigantic „battery“ in which the electric energy produced from inconsistent alternative energy sources, such as wind or the sun, could be stored.