A medical system (100, 300) comprises internal parts (30, 32) for implantation in a patient and external parts (10, 12, 20). The external parts comprise an energy source (10) with a primary coil (12) and the internal parts comprise an electrically powered medical device (40, 41) and an energy receiver (30) with a secondary coil (32) for inductively receiving energy from the external energy source (10). The system (100, 300) is arranged to determine a balance between the energy received in the energy receiver (30) and the amount of energy used by the medical device (40, 41). The internal control unit (50) transmits feedback information to the external control unit (20), and the system (100, 300) is arranged to determine the feedback information based on a first (P1) and a second (P2) parameter.