An LED security light includes a power supply unit comprising a first power source, a second power source and a power switching circuitry, wherein the power switching circuitry utilizes a detector to detect an available electric power in the first power source or in the second power source, and operates with the available electric power detected as a criterion to selectively connect an LED load to at least one of the first power source and the second power source for performing an illumination mode in responding to signals generated by a light sensing unit, a motion sensor unit, or at least one external control unit. In an exemplar, the first power source is an AC power source or a solar power module, and the second power source is a battery unit or a rechargeable battery module.