A charging system for an electric window covering, which includes a motorized assembly controlling the movement of the electric window covering, and a power supply device including a rechargeable power unit and a first remaining power detection circuit is disclosed. The charging system comprises a charging device which provides power to the power supply device, a comparison module, and a notification unit. The first remaining power detection circuit detects the potential magnitude of the power unit, and the detected value is transmitted to the comparison module to be compared. If the potential magnitude is less than a predetermined threshold, a warning is presented at a notification unit. By actively presenting the warning to the user when the power of the power unit is low, the power supply device and the charging system can achieve the objectives of providing an active notification and extending the run-time of the power unit.