A GPU (Ground Power Unit) is a generator that is connected to an aircraft when it arrives at the gate and supplies electrical power to the aircraft. Usually the term GPU refers to a mobile generator that is towed to the aircraft and then towed away once it is no longer required and has been disconnected. FEP (Fixed Electrical Power) usually refers to a system that is either connected from the airbridge or from the ground and is fixed in place and is not removed.