The invention proposes a method of calculating power loss in an inductive power transfer system comprising a power transmitter (112) for transmitting power inductively to a power receiver (110) via transmitter coil (114) and receiver coil (104), the method comprising a step of obtaining, by power transmitter, time information for time alignment to enable the power transmitter to align the time of calculating the power loss with the power receiver; and a step of calculating power loss during power transfer according to the obtained time information and received power parameter communicated from the power receiver.