An uninterruptible power supply apparatus comprising: a converter configured to convert AC power supplied from a commercial AC power source into DC power; and an inverter configured to convert DC power into AC power and supply the converted power to a load, during a normal time when AC power is supplied from the commercial AC power source, DC power generated in the converter being supplied to the inverter and stored in a power storage device, and during a power failure time when a supply of AC power from the commercial AC power source is stopped, DC power in the power storage device being supplied to the inverter, the load being configured to receive an AC voltage within a range of allowable input voltage to consume constant AC power, the uninterruptible power supply apparatus having maximum efficiency when a ratio of load capacity to rated capacity of the uninterruptible power supply apparatus is a predetermined value, the uninterruptible power supply apparatus further comprising a control device configured to control an output voltage of the inverter within the range of allowable input voltage so as to increase the efficiency, when the ratio of the load capacity to the rated capacity is different from the predetermined value, the control device being configured to decrease the output voltage of the inverter within the range of allowable input voltage of the load and increase a load current so as to increase the efficiency, when the ratio of the load capacity to the rated capacity is lower than the predetermined value, the control device being configured to increase the output voltage of the inverter within the range of allowable input voltage and decrease a load current so as to increase the efficiency, when the ratio of the load capacity to the rated capacity is higher than the predetermined value. a converter configured to convert AC power supplied from a commercial AC power source into DC power; and an inverter configured to convert DC power into AC power and supply the converted power to a load, during a normal time when AC power is supplied from the commercial AC power source, DC power generated in the converter being supplied to the inverter and stored in a power storage device, and during a power failure time when a supply of AC power from the commercial AC power source is stopped, DC power in the power storage device being supplied to the inverter, the load being configured to receive an AC voltage within a range of allowable input voltage to consume constant AC power, the uninterruptible power supply apparatus having maximum efficiency when a ratio of load capacity to rated capacity of the uninterruptible power supply apparatus is a predetermined value, the uninterruptible power supply apparatus further comprising a control device configured to control an output voltage of the inverter within the range of allowable input voltage so as to increase the efficiency, when the ratio of the load capacity to the rated capacity is different from the predetermined value, the control device being configured to decrease the output voltage of the inverter within the range of allowable input voltage of the load and increase a load current so as to increase the efficiency, when the ratio of the load capacity to the rated capacity is lower than the predetermined value, the control device being configured to increase the output voltage of the inverter within the range of allowable input voltage and decrease a load current so as to increase the efficiency, when the ratio of the load capacity to the rated capacity is higher than the predetermined value.