In a method and apparatus for supplying wireless energy to a medical device (100) implanted in a patient, wireless energy is transmitted from an external energy source (104) located outside a patient and is received by an internal energy receiver (102) located inside the patient, for directly or indirectly supplying received energy to the medical device. An energy balance is determined between the energy sent by the external energy source and energy received by the internal energy receiver, and the transmission of wireless energy is then controlled based on the determined energy balance. The energy balance thus provides an accurate indication of the correct amount of energy needed, which is sufficient to operate the medical device properly, but without causing undue temperature rise.