An assembly for an aircraft includes a power system and a monitoring system. The power system includes an electric power source, an electric motor and a motor controller electrically coupled between the electric power source and the electric motor. The motor controller is configured to regulate a flow of electricity from the electric power source to the electric motor. The electric motor is configured to drive rotation of a rotating component. The monitoring system is configured to determine an electric power output parameter, a mechanical power output parameter and an efficiency parameter based on the electric power output parameter and the mechanical power output parameter. The electric power output parameter is indicative of an electric power drawn by the electric motor through the motor controller from the electric power source. The mechanical power output parameter is indicative of a mechanical power received by the rotating component from the electric motor.