Is the energy through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy Since olden days it has been used to move mills, sawmills and cranes and nowadays to produce electrical energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuelpowered energy plants. Worldwide, an installed capacity of 777 GWe supplied 2998 TWh of hydroelectricity in 2006.[1] This was approximately 20% of the world's electricity, and accounted for about 88% of electricity from renewable sources.