Water energy as well as most of the other RES comes into existence due to solar activity that foster evaporation from seas, oceans and water surafaces, consequently formation of water vapour, clouds and return back to Earths surface in the form of precipitation, which creates closed hydrological cycle. Energy of water can be used for power production in hydro power plants (HPPs). A clear advantage of HPPs is their flexibility as well as the fact that they do not emit any harmful emmissions (including GHG) in their power generation. The most appropriate environmental sources are considered hydro power plants with lower installed capacity, the so called small-scale hydro power plants. Their classification is non-uniform at the present. Small-scale HPPs in India and China are considered the facilities with the installed capacity of up to 25 MW while in Latin America the mark is even higher - 30 MW. Although in the European Union the level was set at 10 MW, in some member states the limit is even lower (5 MW in Germany and 3 MW in Italy).
World-wide potential of hydro energy has been estimated at 4.6 x 1013 kWh. European leaders in the sector of hydro energy (specifically small-scale hydro power plants) are Italy, France, Spain, Austria and Sweden. Among the new member states the biggest progress in the recent years has been registered in the Czech Republic, Poland and Slovenia.
In Slovakia small-scale HPPs generated 250 GWh of electricity in 2005, which was accomplished with the cumulated installed capacity of 63 MW. According to official studies and strategic documents setting the basic goals and frameworks of the development of energetics through a long-term perspective (Strategy for advanced use of renewable energy sources in the Slovak Republic; proposal on Strategy of energy security of the Slovak Republic up to 2030) the goal is to reach 350 GWh of power generated in small-scale HPPs by 2010 and 450 GWh by 2015.