Automatic generation control (AGC) of power generation units aims at providing satisfactory responses of generated active powers to desired active powers dispatched from a power grid center. This paper proposes a method to estimate two frequently-used AGC performance metrics of response rapidity and accuracy. The proposed method is composed of two main  parts . The first part selects step-like data segments as those being similar to a designed step-change time sequence, based on consecutive piece-wise linear representations of the desired active power.