In a diagnosing method for an apparatus having movable mechanical components and a diagnosing device, an instantaneous total supply of power is measured and recorded as a function of time. A dynamic physical power-flow model of the driven apparatus is created. The model is described by means of parameters directly connected to mechanical and/or electromagnetic quantities of the individual components of the apparatus. The parameters are adapted to fit the power consumption calculated from the model to the recorded power signatures. A diagnosis is performed based on the adapted parameters. The diagnosing device has a power meter and a processor, the processor being arranged for: obtaining a power signal representing supplied power as a function of time; providing the dynamic power-flow model; and adapting values of model parameters of the dynamic physical power-flow model to fit signatures of said power signal and for diagnosing the apparatus.