In the evolving landscape of power system operations, maintaining stability becomes increasingly crucial for system operators. In this context, the rapid response capabilities of Battery Energy Storage Systems (BESS) are recognized as a resource that can effectively contribute to the stability of the power system. However, to evaluate the performance of BESS, an effective method is essential. This paper proposes an approach to assess the frequency response performance of BESS by quantitatively comparing it with the governor of conventional generators. Through this quantification, it becomes possible to reflect the contribution of BESS to securing reserve power and maintaining the stability of the power system.