Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable 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 fuel powered energy plants. Worldwide, an installed capacity of 1,010 GW supplied hydroelectricity in 2010. Approximately 16% of the world's electricity is renewable, with hydroelectricity account for 21% of renewable sources and 3.4% of total energy sources.