Modern pedelecs (electric bicycles) run on brushless electric motors that draw power from a battery contained in the bicycle frame or on the bicycle rack. They are available in three configurations; the two currently most popular are front or rear hub motors. The third kind is the crank drive or centre motor.
The centre motor works on the crank axle, augmenting the cyclist’s efforts. Crank motors work through the bicycle’s transmission system (gears) and are more efficient when using the gears to climb hills.