An uninterruptable power supply (UPS) designed to switch power inputs between an alternating current mode and a battery mode having a sensor to monitor a load current, and a control processing unit (CPU) to boost a bus voltage of the uninterruptable power supply by increasing a calculated reference voltage to a target reference voltage by a slope. The calculated reference voltage is calculated by the CPU based on a voltage drop value in main capacitors that occurs during a transfer time between the battery mode and the AC mode. The target reference voltage is the voltage across an equivalent circuit of the main capacitors before the voltage drop. The slope is based on a ratio of a voltage drop value across an equivalent circuit of main capacitors and a charging up time to reach the target reference voltage.