In this bachelor thesis, hydropower plants are briefly described, applications and divisions of pumped storage hydropower plants and their impact on the cost of units produced in the power system and the electricity market. The second chapter describes the various divisions of hydroelectric power plants. The most imortant division is according to the way of using water is used, i.e. flow regulation. According to this division, hydropower plants are divided into: storage, flow and reversible hydropower plants. Parts of the hydroelectric power plant and the hydroelectric power plant in the Republic of Croatia are also listed and described. The third chapter describes the characteristic of pumped storage hydropower plants and their mode of operation, that allows energy storage during low energy prices and use of that energy during high energy prices. In the Republic of Croatia we have two pumped storage hydropower plants: Velebit and Lepenica. The fourt chapter describes the operations on electricity exchanges in the Republic of Croatia and Europe, which is standardized, as well as the contracts traded on the stock exchange, the method of calculation, payment and disbursement. The fifth chapter compares energy prices for the day-ahead for the UK market, as well as prices for ancillary services regulation up and regulation down for the S3 Nordpool area from 2019, which we consider the year before the COVID-19 crisis to 2021, which we consider the year of recovery from COVID-19 crisis.