Battery energy storage system (BESS) is attracting attention all over the world. BESS yields two types of benefits, i.e., economical benefit and environmental benefit. Economical benefit is yielded by reducing the cost of power plants that is composed by the operation cost and the construction cost. Recent serious concern against environment will be more important in the near future and it might be treated as penalty and/or constraints. The installation merit of BESS stands out when load factor of demand is low. If base generators composed of inexpensive and clean sources, and if peak generators composed of expensive and dirty sources, the benefit against operation cost becomes more remarkable. Besides, BESS can contribute to developing countries by reducing construction cost of generation plants. In this paper, the numerical analysis of this benefit is presented. The optimal amount of BESS is searched by evaluating CO2-reductions and cost benefits.