Agriculture requires energy as an important input to production. Agriculture uses energy directly as fuel or electricity to operate machinery and equipment, to heat or cool buildings, and for lighting on the farm, and indirectly in the fertilizers and chemicals produced off the farm. In 2002, the U.S. agricultural sector used an estimated 1.7 quadrillion Btu of energy from both direct (1.1 quadrillion Btu) and indirect (0.6 quadrillion Btu) sources. However, agriculture's total use of energy is low relative to other U.S. producing sectors. In 2002, agriculture's share of total U.S. direct energy consumption was about 1%. Agriculture's shares of nitrogen and pesticide use ? two of the major indirect agricultural uses identified by the U.S. Dept of Agriculture (USDA) ? are signficantly higher at about 56% and 67%, respectively.