Taking the electric energy storage system applied in the power system as the research object and based on the externality theory, the energy storage system economic calculation model considering the externality is constructed, and the energy storage system is connected to the photovoltaic power station as an example. The cash flow analysis method estimates the cost and benefits of energy storage investment, and studies the economics of the energy storage system applied to the renewable energy generation side from the perspective of investors as well as social overall benefits is studied, and the profit and loss balance of energy storage is analyzed. Through the break even curve, this paper analyzes the scenario of energy storage break even is analyzed. The results of economic analysis show that the external income of energy storage system accounts for a large proportion of annual income.