The final paper briefly describes hydroelectric power plants, their applications and divisions, and their impact on the costs of produced units in the EES and on the electricity market. The second chapter describes various divisions of hydroelectric power plants. The most important division is according to the method of water use, i.e. flow regulation. According to this division, hydroelectric power plants are divided into: storage, run-of-river, and reversible. The parts of a hydroelectric power plant are also described and hydroelectric power plants in the Republic of Croatia are listed. The third chapter describes the characteristics of reversible hydroelectric power plants and their operating mode, which allows for the storage of energy during low energy prices and the use of that same energy during high energy prices. In the Republic of Croatia, we have two reversible hydroelectric power plants: Velebit and Lepenica. The fourth chapter describes the standardized operations on electricity exchanges in the Republic of Croatia and Europe, including standardized contracts traded on the exchange, methods of calculation, payments, and disbursements. Chapter 5 compares day-ahead energy prices for the UK market as well as prices for ancillary services for upward and downward regulation for the S3 Nordpool area from 2019, which we consider to be the year before the COVID-19 crisis, to 2021, which we consider to be the year of recovery from the COVID-19 crisis.