A light unit that includes an internal power supply that may be used in the event of an external power failure to provide power to the light unit. In one aspect, the present disclosure provides a lighting apparatus, comprising (a) a power input configured to receive external power from an external power source; (b) a solid state light element that is interconnected to the power input; (c) a detection circuit interconnected to the power input and that, when power is not provided to the power input, measures input impedance at the power input and determines if there is a lack of external power; and (d) a back-up power source that is interconnected to the solid state light element and the power input and that provides power to the solid state light element when the light element is; not provided with power from the power input.