A power supply system for illuminating an LED lamp unit, having a normal mode of operation in which a mains input of a driver of the LED lamp unit is supplied by mains power, and an emergency mode of operation, in which the LED lamp unit is supplied by a charge storage device, is disclosed. The system includes an emergency mode converter operable to step-up the voltage from the charge storage device and to supply the mains driver input for illuminating the LED lamp unit.