In a method for operating a system having an implantable medical device that communicates by wireless telemetry data exchange with an external unit, an algorithm is employed, selecting either an internal calculation processor in the implantable device or an external calculation processor in the external unit for accomplishing a given data processing. If the sum of an estimated amount of energy to process the data in the internal calculation processor and an estimated amount of energy required to then transmit an expected amount of result data to the external unit exceeds an estimated amount of energy required to transmit the source data to the external unit, the external calculation processor is selected. Otherwise, the internal calculation processor is selected. The method thereby results in a minimal energy consumption in the implantable medical device. This, in turn, allows for a device with a long battery lifetime and thus an improved patient comfort.