Emergency lighting for an aircraft has a primary power source for light units provided by the normal aircraft power supply and a secondary power source provided by power supply units operable to back-up the primary power source. Each light unit may have its own power supply unit operable to power the light unit in an emergency if the normal aircraft power supply fails or is otherwise unable to power the emergency lighting. Each power supply unit includes one or more electric double layer capacitors that are charged whenever the normal aircraft power supply is operational to provide the primary power source. The useful life of the electric double layer capacitors may be prolonged by reducing the capacitor voltage when the emergency lighting is switched off.