Either Diesel engines or electric motors can function as prime movers. The power can range from under 7.5 kW (10 hp) for the smallest of lifts, to more than 750 kW (1000 hp) for a long, swift, detachable eight-seat up a steep slope. DC electric motors and DC drives are the most common, though AC motors and AC drives are becoming economically competitive for certain smaller chairlift installations. DC drives are less expensive than AC variable-frequency drives and were used almost exclusively until the 21st century when costs of AC variable-frequency drive technology dropped. DC motors produce more starting torque than AC motors, so applications of AC motors on chairlifts is largely limited to smaller chairlift installations, otherwise the AC motor would need to be significantly oversized relative to the equivalent horsepower DC motor.