In order to rise, an elevator must expend at least enough energy to lift itself and its occupants to the desired height against the downward pull of gravity. Most elevator systems consume approximately 5 percent of a building's energy budget, according to the American Council of Energy Efficient Economy
The energy lost to friction varies enormously by elevator design. The American Society of Mechanical Engineers (ASME) describes energy-saving innovations, such as double-decker cabs that can serve even- and odd-numbered floors simultaneously, and greatly reduce the energy lost to multiple winches and drive mechanisms. Regenerative drives further reduce waste by reclaiming the energy of an elevator as it brakes and then using it to drive the elevator, rather than permitting the cab's kinetic energy to be lost as heat.