Water power (hydropower) is related to obtaining water energy and its conversion into mechanical energy and electricity by means of water engines (water turbines) and hydro generators in hydropower plants (such as mills), and water power stations, as well as other facilities (ocean thermal energy conversion facilities and wave power plants). Hydropower is based primarily on the use of energy of inland waters (less often seas, e g. in tidal power plants) with high flow rate and a significant fall - measured by the difference in water levels of the upper and bottom water taking into account the flow losses.
The potential energy of watercourses is commonly used. In the past, hydropower was used only locally to drive simple mechanical devices, mills and sawmills. At present, this energy is converted into electricity transmitted to the power grid or used locally.
In Poland, lower Vistula and Dunajec are of greatest importance in respect of hydropower.
Recently, an increasing attention has been paid to the use of energy of small watercourses through the construction of the so-called small water power plants; in the first place, this concerns those courses where there already exist swelling devices used for other purposes. One thing that weighs up in favour of the development of hydropower is the fact that the cost of the electricity produced by water power plants is lower than the electricity produced by thermal power plants.