The cost of bringing transmission mains from a country's main electric grid to rural areas is often prohibitive. Diesel generators are expensive and other forms of electricity generation are not feasible on a wide scale. In many developing countries there is potential for using small hydroelectric stations, unconnected to the main grid, to provide power to the community. Hydroelectric stations supply about 20 percent of the world's electricity demand. But only about 30 percent of potential has been exploited to date, and most of the remaining potential is in the developing world (Goldsmith, 1989).