Abstract: Micro grid is an important part of a smart grid, and it can effectively coordinate the power resources in the region. To research the power cost change and optimization of the whole micro grid for energy storage, based on the multi agents structure of photovoltaic micro grid, an equalization cost model for power resources has been established. Under the condition of the statistical data of power supply and demand quantity in photovoltaic micro grid, a profit function of distributed power supply, a cost function of power demands and a cost function of power purchase were created by choosing time as the independent variable. Through analyzing the cost function, the dynamic change of the power equilibrium dispatching in photovoltaic micro grid has been presented. Based on a practical example of the micro grid model, it was found that the cost of micro grid in the grid-connected scenario was less than that in the energy storage scenario(Cconnected=2033.6 Yuan< Cshortage=2633.6 Yuan),and 22.8% of the cost under energy storage scenario came from the diurnal depreciation of energy storage facilities. Therefore, intensifying R&D and strengthening the collaborative management in micro grid could reduce the cost of micro grid.