A wireless power receiver comprising: a circuit comprising a tunable capacitor C 2 in parallel with an inductor L 1, the wireless power receiver adapted to receive an induced voltage input V induced at the inductor L 1 due to a magnetic field generated by a wireless power transmitter, the wireless power receiver having an effective series resistance R receiver; a rectifier coupled to the circuit, the rectifier having an output with a rectified voltage (V rect) and a rectified current I rect; and a controller having a first input for receiving a signal representative of the rectified voltage (V rect) and a first output for supplying an adjustment signal to the tunable capacitor C 2, the controller including a processor coupled to the first input and configured to: (i) operate on the signal representative of rectified voltage (V rect) to produce a desired capacitance value C 2_desired for capacitor C 2; (ii) provide the adjustment signal determined so as to adjust a capacitance value C 2_value of the tunable capacitor C 2 to the desired capacitance value C 2_desired, characterized in that, to produce the desired capacitance value C 2_desired, the controller is configured to: (iii) receive, at a second input of the controller, a signal representative of a rectified current I rect of the rectifier output; (iv) operate, in the processor, on the signals representative of each of the rectified voltage (V rect) and current lrect to determine power P L to a load coupled to the rectifier output and resistance R L at the load; (v) operate, in the processor, on the receiver resistance R receiver and power P L to determine power P receiver to the receiver; and (vi) operate, in the processor, on power P receiver, to determine an open circuit voltage value V open, wherein the desired capacitance value C 2_desired is a function of the open circuit voltage value V open. a circuit comprising a tunable capacitor C 2 in parallel with an inductor L 1, the wireless power receiver adapted to receive an induced voltage input V induced at the inductor L 1 due to a magnetic field generated by a wireless power transmitter, the wireless power receiver having an effective series resistance R receiver; a rectifier coupled to the circuit, the rectifier having an output with a rectified voltage (V rect) and a rectified current I rect; and a controller having a first input for receiving a signal representative of the rectified voltage (V rect) and a first output for supplying an adjustment signal to the tunable capacitor C 2, the controller including a processor coupled to the first input and configured to: (i) operate on the signal representative of rectified voltage (V rect) to produce a desired capacitance value C 2_desired for capacitor C 2; (ii) provide the adjustment signal determined so as to adjust a capacitance value C 2_value of the tunable capacitor C 2 to the desired capacitance value C 2_desired, characterized in that, to produce the desired capacitance value C 2_desired, the controller is configured to: (iii) receive, at a second input of the controller, a signal representative of a rectified current I rect of the rectifier output; (iv) operate, in the processor, on the signals representative of each of the rectified voltage (V rect) and current lrect to determine power P L to a load coupled to the rectifier output and resistance R L at the load; (v) operate, in the processor, on the receiver resistance R receiver and power P L to determine power P receiver to the receiver; and (vi) operate, in the processor, on power P receiver, to determine an open circuit voltage value V open, wherein the desired capacitance value C 2_desired is a function of the open circuit voltage value V open. (i) operate on the signal representative of rectified voltage (V rect) to produce a desired capacitance value C 2_desired for capacitor C 2; (ii) provide the adjustment signal determined so as to adjust a capacitance value C 2_value of the tunable capacitor C 2 to the desired capacitance value C 2_desired, characterized in that, to produce the desired capacitance value C 2_desired, the controller is configured to: (iii) receive, at a second input of the controller, a signal representative of a rectified current I rect of the rectifier output; (iv) operate, in the processor, on the signals representative of each of the rectified voltage (V rect) and current lrect to determine power P L to a load coupled to the rectifier output and resistance R L at the load; (v) operate, in the processor, on the receiver resistance R receiver and power P L to determine power P receiver to the receiver; and (vi) operate, in the processor, on power P receiver, to determine an open circuit voltage value V open, wherein the desired capacitance value C 2_desired is a function of the open circuit voltage value V open.