An aspect of this disclosure is an apparatus for receiving power wirelessly. The apparatus comprises a power receiver circuit that receives power from a magnetic field of a transmitter to provide to a load. At least one receiver component is coupled with the power receiver circuit and operates based on at least one operation parameter. A sensor measures at least one of a current and a voltage at the load. A controller estimates a first voltage induced by the magnetic field based on the at least one measured current and measured voltage and the at least one operation parameter. The controller also estimates a second voltage based on the at least one operation parameter, the second voltage corresponding to a voltage at which the power receiver circuit operates with an efficiency level that exceeds a threshold efficiency. The communication circuit communicates the estimated voltages to the transmitter.