An apparatus for a wireless power transfer system including a power receiver for receiving a power transfer from a power transmitter via a wireless inductive power signal; the apparatus comprising: a parasitic power loss detector arranged to perform a parasitic power loss detection during a power transfer phase, the parasitic power loss detection being arranged to indicate a positive detection of a parasitic power loss in response to a parasitic power loss estimate exceeding a first threshold, the parasitic power loss being indicative of a difference between a transmit power for the power transmitter and a receive power for the power receiver; a parameter processor arranged to generate parameter value sets for time instants during the power transfer phase, each parameter value set comprising a set of values indicative of the transmit power of the power transmitter and the receive power of the power receiver for the time instant of the parameter value set; a store for storing the parameter value sets; a foreign object detector for performing foreign object detection tests, each foreign object detection test estimating whether a foreign object is present or not, the foreign object detector arranged to use a different criterion for detecting a presence of a foreign object than the parasitic power loss detector uses for indicating a positive detection; a retriever for retrieving a group of parameter value sets from the store; and a first adapter for adapting the parasitic power loss detection in response to the group of parameter value sets, characterized in the apparatus further comprising a time processor for determining a first time window in response to a foreign object detection test indicating that no foreign object is present, the first time window at least partially preceding the foreign object detection test indicating that no foreign object is present; and in the retriever being arranged to select parameter value sets for time instants within the first time window for the group of parameter value sets. a parasitic power loss detector arranged to perform a parasitic power loss detection during a power transfer phase, the parasitic power loss detection being arranged to indicate a positive detection of a parasitic power loss in response to a parasitic power loss estimate exceeding a first threshold, the parasitic power loss being indicative of a difference between a transmit power for the power transmitter and a receive power for the power receiver; a parameter processor arranged to generate parameter value sets for time instants during the power transfer phase, each parameter value set comprising a set of values indicative of the transmit power of the power transmitter and the receive power of the power receiver for the time instant of the parameter value set; a store for storing the parameter value sets; a foreign object detector for performing foreign object detection tests, each foreign object detection test estimating whether a foreign object is present or not, the foreign object detector arranged to use a different criterion for detecting a presence of a foreign object than the parasitic power loss detector uses for indicating a positive detection; a retriever for retrieving a group of parameter value sets from the store; and a first adapter for adapting the parasitic power loss detection in response to the group of parameter value sets,