The invention relates to uninterruptible power supply devices which make it possible to supply emergency power to a load, which is to be supplied, in the event of mains failure. The invention is based on the object of providing a device for supplying uninterruptible power, which device can be operated in a more energy-saving manner and in which the emergency power can be controlled in a rapid and safe manner. To this end, the uninterruptible power supply device has a power transistor which can be rapidly switched and can be used to connect a standby power source to the output connections during emergency power operation. The output current can also be dynamically limited by driving the power transistor, which can be rapidly switched, in a corresponding manner. In addition, the input and output of the UPS device are decoupled using a field effect transistor that acts as a power transistor.