An apparatus for wirelessly transferring power via a magnetic field in a wireless power transfer system, the system including a receiver having a resonant receive circuit configured to inductively couple power via the magnetic field, a power conversion circuit (680, 655) of the receiver configured to adjust an amount of reactive power in the wireless power transfer system to maintain an output power substantially constant in response to changes in the power received due to variations in coupling, the apparatus comprising: an inverter circuit configured to output alternating electrical current; a resonant transmit circuit comprising a coil electrically connected to a capacitive element, the resonant transmit circuit operably coupled to the inverter circuit and configured to generate the magnetic field in response to being driven with the alternating electrical current; and a controller characterised in that it is configured to: receive a value indicative of load angle of the inverter circuit; and adjust a duty cycle of the inverter circuit to trigger a change in an electrical current in the receive circuit, the amount of adjustment of the duty cycle of the inverter selected by the controller to cause adjustment of an amount of reactive power in the wireless power transfer system based on a change in a duty cycle of a switching network associated with the power conversion circuit that automatically occur to maintain the output power substantially constant in response to the change in the electrical current in the resonant receive circuit. an inverter circuit configured to output alternating electrical current; a resonant transmit circuit comprising a coil electrically connected to a capacitive element, the resonant transmit circuit operably coupled to the inverter circuit and configured to generate the magnetic field in response to being driven with the alternating electrical current; and a controller characterised in that it is configured to: receive a value indicative of load angle of the inverter circuit; and adjust a duty cycle of the inverter circuit to trigger a change in an electrical current in the receive circuit, the amount of adjustment of the duty cycle of the inverter selected by the controller to cause adjustment of an amount of reactive power in the wireless power transfer system based on a change in a duty cycle of a switching network associated with the power conversion circuit that automatically occur to maintain the output power substantially constant in response to the change in the electrical current in the resonant receive circuit. receive a value indicative of load angle of the inverter circuit; and adjust a duty cycle of the inverter circuit to trigger a change in an electrical current in the receive circuit, the amount of adjustment of the duty cycle of the inverter selected by the controller to cause adjustment of an amount of reactive power in the wireless power transfer system based on a change in a duty cycle of a switching network associated with the power conversion circuit that automatically occur to maintain the output power substantially constant in response to the change in the electrical current in the resonant receive circuit.