A power supply apparatus 4, which can be a lighting ballast for use with plant grow lighting, comprises a control means for supplying power to an output terminal and operable in an ON state in which power is supplied at a first level and an OFF state in which power is supplied at second, lower, level. The apparatus also comprises an in-built timing means configured in use to cycle the control means between the ON and OFF states for different predetermined ON/OFF timing periods dependent on which one of a plurality of user-selectable modes is selected.
