A power adaptor (20) for a lighting unit (50) having a solid state light source (50a,50b,50c) is disclosed. The power adaptor (20) comprises an input (22) for connection to a mains power supply, a power transfer module (40a,40b,40c,140a) that is coupled to the input (22) and provides an output suitable for driving the solid state light source (50a,50b,50c), and a controller (30) that receives a voltage signal from the input (22) and is able to deliver a control signal to the power transfer module (40a,40b,40c,140a) for reducing the power drawn from the input (22). The power adaptor (20) draws current from the input (22) as a function of the voltage at the input (22) in order that the power adaptor (20) appears as a variable resistor to the mains supply.