A regenerative drive is a variable-frequency drive that returns braking energy to the building supply instead of dissipating it as heat in a resistor. An elevator generates energy whenever it runs overhauling, that is a full car descending or an empty car ascending, and a regenerative unit converts this back to clean mains-synchronised AC. Typical savings are 20 to 40 percent of lift energy consumption, with the largest benefit on high-rise and high-traffic installations. Regeneration also reduces machine room heat load and air-conditioning cost.