An uninterruptible electric power system comprising: a main power unit configured to receive AC commercial power; an uninterruptible power supply unit configured to receive AC commercial power from the main power unit and configured to convert the AC commercial power to converted AC power; and a slave power unit connected to the uninterruptible power supply unit configured to receive the converted AC power from the uninterruptible power supply unit, the slave power unit comprising a slave static transfer switch, characterised in that the main power unit comprises a first automatic transfer switch and the slave power unit further comprises a second automatic transfer switch, wherein the first automatic transfer switch comprises a first main input terminal, a first slave input terminal, and a first output terminal, the first main input terminal is configured to receive the AC commercial power, the first slave input terminal is connected to a generator, the first output terminal is connected to the uninterruptible power supply unit, the first output terminal is configured to be coupled to the first main input terminal when the AC commercial power is on, and to be coupled to the first slave input terminal when the AC commercial power is off and the generator works; wherein the slave static transfer switch comprises: a first input terminal connected to the output of the second automatic transfer switch, a second input terminal configured to receive the AC commercial power, and an output terminal configured to connect, when the uninterruptible power supply unit is in normal operation, to the first input terminal to receive the converted AC power, and, when the uninterruptible power supply unit operates abnormally, to the second input terminal to receive the AC commercial power; wherein the second automatic transfer switch comprises a second main input terminal, a second slave input terminal, and a second output terminal, the second main input terminal is connected to the uninterruptible power supply unit, the second slave input terminal is connected to the generator, the second output terminal is connected to the first input terminal of the slave static transfer switch, the second output terminal is configured to be coupled to the second main input terminal when the second main input terminal receives the converted AC power, and to be coupled to the second slave input terminal when the second main input terminal is not receiving the converted AC power and the generator works. a main power unit configured to receive AC commercial power; an uninterruptible power supply unit configured to receive AC commercial power from the main power unit and configured to convert the AC commercial power to converted AC power; and a slave power unit connected to the uninterruptible power supply unit configured to receive the converted AC power from the uninterruptible power supply unit, the slave power unit comprising a slave static transfer switch, characterised in that the main power unit comprises a first automatic transfer switch and the slave power unit further comprises a second automatic transfer switch, wherein the first automatic transfer switch comprises a first main input terminal, a first slave input terminal, and a first output terminal, the first main input terminal is configured to receive the AC commercial power, the first slave input terminal is connected to a generator, the first output terminal is connected to the uninterruptible power supply unit, the first output terminal is configured to be coupled to the first main input terminal when the AC commercial power is on, and to be coupled to the first slave input terminal when the AC commercial power is off and the generator works; wherein the slave static transfer switch comprises: a first input terminal connected to the output of the second automatic transfer switch, a second input terminal configured to receive the AC commercial power, and an output terminal configured to connect, when the uninterruptible power supply unit is in normal operation, to the first input terminal to receive the converted AC power, and, when the uninterruptible power supply unit operates abnormally, to the second input terminal to receive the AC commercial power; wherein the second automatic transfer switch comprises a second main input terminal, a second slave input terminal, and a second output terminal, the second main input terminal is connected to the uninterruptible power supply unit, the second slave input terminal is connected to the generator, the second output terminal is connected to the first input terminal of the slave static transfer switch, the second output terminal is configured to be coupled to the second main input terminal when the second main input terminal receives the converted AC power, and to be coupled to the second slave input terminal when the second main input terminal is not receiving the converted AC power and the generator works.