A power transmitter for a wireless power transfer system including a power receiver for receiving a power transfer from the power transmitter via a wireless inductive power signal; the power transmitter comprising: a transmit power inductor for generating the wireless inductive power signal; a first detector arranged to generate, when in a test mode, a foreign object detection estimate in response to a comparison of a first measured load of the wireless inductive power signal to an expected load of the wireless inductive power signal when the power receiver is operating in a test mode, a loading of the wireless inductive power signal by the power receiver when operating in the test mode being constrained relative to the loading of the wireless inductive power signal by the power receiver when operating in a power transfer mode during a power transfer phase; a controller for entering at least one of the power transmitter and the power receiver into a power transfer mode when the foreign object detection estimate indicates that no foreign object is present; a second detector arranged to generate a parasitic power loss detection for the power transfer in response to a parasitic power loss estimate exceeding a threshold; characterized in that the second detector is arranged to generate the parasitic power loss detection when in the power transfer mode, and in that the power transmitter further comprises: a calibration unit for initializing an adaptation of a first parameter of the parasitic power loss detection in response to operating parameter values for the power transfer phase during an initial time interval of entering the power transfer phase following the foreign object detection estimate indicating that no foreign object is present and to terminate the adaptation of the parameter while remaining in the power transfer phase, wherein the calibration unit is arranged to receive a plurality of receive power estimates from the power receiver during the initial time interval, the plurality of receive power estimates providing indications of a power received by the power receiver for different loads of the power receiver; and the calibration unit is arranged to adapt a plurality of parameters of the parasitic power loss detection in response to the plurality of receive power estimates. a transmit power inductor for generating the wireless inductive power signal; a first detector arranged to generate, when in a test mode, a foreign object detection estimate in response to a comparison of a first measured load of the wireless inductive power signal to an expected load of the wireless inductive power signal when the power receiver is operating in a test mode, a loading of the wireless inductive power signal by the power receiver when operating in the test mode being constrained relative to the loading of the wireless inductive power signal by the power receiver when operating in a power transfer mode during a power transfer phase; a controller for entering at least one of the power transmitter and the power receiver into a power transfer mode when the foreign object detection estimate indicates that no foreign object is present; a second detector arranged to generate a parasitic power loss detection for the power transfer in response to a parasitic power loss estimate exceeding a threshold; characterized in that the second detector is arranged to generate the parasitic power loss detection when in the power transfer mode,