A power adaptor to supply power for a portable device has an input circuit (21) for receiving mains input power, a power switch device (25), a power inductance (24) and an output circuit (22) coupled to the power inductance to provide the supply power for the portable device, and a controller (26) for controlling the power switch device according to a supply power requirement of the portable device. The adaptor has a measurement inductance (27) magnetically coupled to the power inductance, and the controller comprises a measurement input (28) for detecting a measurement signal indicative of a magnetic state of the power inductance. The controller detects the supply power requirement based on the magnetic state in response to a controlling of the power switch device via the controller. Advantageously different portable devices can be supplied by appropriate power.