Power capping apparatus and method

A power capping apparatus including a measurement unit to measure a performance counter value and a used amount of power of the computing system before a power limit value is set. A calculation unit to calculate an energy reference value used in an energy conservation mode using the used amount of power and the performance counter value. A management unit to compare a first used amount of power measured before a power limit value is set with the power limit value when the power limit value is set, and limit the used amount of power to a value below the power limit value when the first used amount of power is greater than the power limit value, wherein the management unit outputs an error message so that a user sets the power limit value in which the energy reference value is within an effective range in the energy conservation mode.