Fossil fuels are the largest energy source in the United States. Petroleum, coal and natural gas account for about 40%, 23% and 24% of the total energy consumed in the U.S., respectively. Petroleum is used primarily by the transportation sector. Coal is used primarily for electricity generation, producing approximately 50% of generated electricity . Approximately 70% of natural gas is used in equal parts by chemical industries and by commercial/residential consumers, while the remaining natural gas is used to produce 18% of U.S. electricity. 20% of electricity is also generated using nuclear energy. Renewable resources are a minor energy source in the U.S. generating less than 10% of U.S. electricity, with hydroelectric power being the largest contributor to this percentage.