The power fluctuation and instability problems caused by wind and solar power generation can easily lead to source-load imbalance in the system, which poses a significant challenge to the energy management of DC microgrids. Therefore, this paper proposes a power calculation strategy for DC microgrid converter systems based on the extended simplex method. Firstly, an optimization model for typical operating conditions in on-grid and off-grid scenarios is developed based on DC microgrid systems including wind, solar, energy storage and fuel cells. Secondly, the simplex method is extended by adding slack variables and penalty terms. Strategies for power calculation are proposed. Finally, a highway service area in western China is used as an example for the calculation. The results show the effectiveness of the proposed strategy and the optimal solution can be identified within 20ms.