A wireless power transfer system includes a power transmitter (101) providing power to a power receiver (105) via an inductive power signal. The power transmitter (101) and receiver (105) can operate in different modes including a test mode and a power transfer mode. Operating parameters of the power receiver (105) are constrained in the test mode relative (and specifically the loading). A foreign object detector (209) generates a foreign object detection estimate from a comparison of a measured load to an expected load of the inductive power signal when the power receiver is operating in the test mode. A controller (211) enters the power transmitter (101) and receiver (103) into the power transfer mode when the foreign object detection estimate is indicative of no detection of a foreign object. In the power transfer mode, a parasitic power loss detector (207) generates a parasitic power loss detection for the power transfer if a parasitic power loss estimate is outside a range.