Provided is a method and apparatus for managing the power of a portable computer system, in which a convenient user interface is provided. The method includes calculating a range of a desired time-of-use based on power consumption of the portable computer system and a remaining battery capacity at a minimum power level of the portable computer system, providing the calculated range of the desired time-of-use and a desired time input box that allows a user to input the desired time-of-use, inputting the desired time-of-use through the desired time input box, and resetting the power level of the portable computer system according to the input desired time-of-use.