The major energy sources in the United States are petroleum (oil), natural gas, coal, nuclear, and renewable energy. The major users are residential and commercial buildings, industry, transportation, and electric power generation. The pattern of fuel use varies widely by sector. For example, oil provides 94% of the energy used for transportation, but only 1% of the energy used to generate electric power. Understanding the relationships between the different energy sources and their uses provides insights into many important energy issues.