Fuel switching is the substitution of one energy source for another in order to meet requirements for heat, power, and/or electrical generation. Generally, this term refers to the practices of some industries that can substitute among natural gas, electricity, coal, and LPG within 30 days without modifying their fuel-consuming equipment and that can resume the same level of production following the change. The U.S. Department of Energy estimates that among manufacturers in 1991, 2.8 quadrillion Btus of energy consumption could be switched from a total consumption of about 20.3 quadrillion Btus, representing about 14%. Price is the primary reason for fuel switching; however, additional factors may include environmental regulations, agreements with energy or fuel suppliers, and equipment capabilities.