An inductive power transfer system comprises a primary side unit (1) and a secondary side unit (6) with a non-conductive barrier (5) therebetween, the primary side unit having a power supply (2), a transmission coil (4) and a controller (12). The secondary side unit has a receiver coil (7), a controller (8) and a load (9). The secondary side controller (8) is configured to detect the voltage in the receiver coil (7) and to transmit to the primary side controller (12) a control signal requesting an increase or decrease in the power transmitted from the primary side according to whether the voltage in the receiver coil (7) is above or below a predetermined value.