Different sources of energy have been used to run generators , including coal , nuclear , natural gas , hydroelectric , and petroleum power and a small amount from solar energy , tidal harnesses , wind generators , and geothermal sources . The first power plants were run on water power or coal , and today we rely mainly on coal , nuclear , natural gas , hydroelectric , and petroleum with a small amount from solar energy , tidal harnesses , wind generators , and geothermal sources .