An inductive wireless power transmitter is presented for transmitting an inductive wireless power signal to at least two inductive wireless power receivers. The power transmitter coordinates communication with the power receivers in time slots. It allocates a measurement time slot for synchronised power measurement by the power transmitter and the power receivers. During the measurement time slot the power transmitter measures the amount of power it transmits. After the end of the measurement time slot it receives from the power receivers the measured amounts of power received. From the measured power transmitted and power received, it determines an amount of power lost. If the amount of power lost exceeds a threshold value, it reduces the power of the inductive wireless power signal.