The cost of hydroelectricity is relatively low, making it a competitive source of renewable electricity. The hydro station consumes no water, unlike coal or gas plants. The typical cost of electricity from a hydro station larger than 10 megawatts is 3 to 5 US cents per kilowatt hour. With a dam and reservoir it is also a flexible source of electricity, since the amount produced by the station can be varied up or down very rapidly (as little as a few seconds) to adapt to changing energy demands. Once a hydroelectric complex is constructed, the project produces no direct waste, and it generally has a considerably lower output level of greenhouse gases than photovoltaic power plants and certainly fossil fuel powered energy plants (see also Life-cycle greenhouse-gas emissions of energy sources). However, when constructed in lowland rainforest areas, where inundation of a part of the forest is necessary, they can emit substantial amounts of greenhouse gases.