An uninterruptible power supply includes a switching unit, a charging unit, an energy storage unit, a conversion unit, and a control unit. When an input power source is available, the switching unit provides a first output power source and the charging unit converts the input power source into a first power source to provide the first power source to the control unit and to charge the energy storage unit. When the input power source is not available, the conversion unit converts the electricity of the energy storage unit into a second output power source and provides the second output power source. When the input power source is not available, the conversion unit provides a second power source to the charging unit so that the charging unit converts the second power source into the first power source and provides the first power source to the control unit.