A system and method of controlling inductive power transfer in an inductive power transfer system with power accounting. Parasitic metal in proximity to the primary unit can be more accurately detected by accounting for changes in known power losses during operation. The amount of power loss during inductive power supply transfer in an inductive power supply system can vary depending on the alignment of the primary unit and the secondary device. The amount of power loss during inductive power supply transfer can also vary as a function of changes in the operating frequency of the switching circuit in the primary unit or as a function of changes in the secondary device load.