An uninterruptible power supply comprising: a battery unit configured to be connected to an external grid to receive electricity from the grid and configured to be connected to an external load to supply electricity to the load, the battery unit comprising a plurality of battery racks each battery rack comprising a rack battery management system, the battery racks being divided into at least one first battery rack comprising a first battery and at least one second battery rack comprising a second battery; an overall control unit configured to determine an operation mode of the battery unit from among a discharge mode and a charge mode, wherein, in the discharge mode, the overall control unit is configured to control the first and second battery racks (210a, 210b) such that the first battery of the first battery rack is discharged to a first reference value of state of charge and the second battery of the second battery rack is discharged to a second reference value of SOC; and a measurement unit configured to monitor an amount of accumulated power consumption of the load during each of a plurality of time periods and transmitting data associated with the monitoring to the overall control unit, wherein the overall control unit is configured to determine whether to operate the battery unit in the discharge mode based on data generated by the measurement unit in each of the time periods, and characterized in that the first reference value of SOC is lower than a high degradation rate band and the second reference value of SOC is higher than the high degradation rate band, wherein the high degradation rate band is defined so that the lifespan decrease rates of the batteries having SOCs in the high degradation rate band when the batteries are not operated are greater than an average lifespan decrease rate of the batteries. a battery unit configured to be connected to an external grid to receive electricity from the grid and configured to be connected to an external load to supply electricity to the load, the battery unit comprising a plurality of battery racks each battery rack comprising a rack battery management system, the battery racks being divided into at least one first battery rack comprising a first battery and at least one second battery rack comprising a second battery; an overall control unit configured to determine an operation mode of the battery unit from among a discharge mode and a charge mode, wherein, in the discharge mode, the overall control unit is configured to control the first and second battery racks (210a, 210b) such that the first battery of the first battery rack is discharged to a first reference value of state of charge and the second battery of the second battery rack is discharged to a second reference value of SOC; and a measurement unit configured to monitor an amount of accumulated power consumption of the load during each of a plurality of time periods and transmitting data associated with the monitoring to the overall control unit, wherein the overall control unit is configured to determine whether to operate the battery unit in the discharge mode based on data generated by the measurement unit in each of the time periods, and characterized in that the first reference value of SOC is lower than a high degradation rate band and the second reference value of SOC is higher than the high degradation rate band, wherein the high degradation rate band is defined so that the lifespan decrease rates of the batteries having SOCs in the high degradation rate band when the batteries are not operated are greater than an average lifespan decrease rate of the batteries.