As the penetration of renewable energy increases a lot, the needs for novel grid topology and control strategy attracts researches. The breakthrough of power electronics makes the power transformation between multiple voltage levels or different current forms easier and cost lower. As for typical commercial building with on-site generation, this paper proposed a novel optimization method for power flow assigned to parallel- connected converters between two voltage levels for DC distribution. By the utilization of Lagrange Multiplier Method, the energy management system for those converters are corrected. In addition, several popular energy efficiency definitions are compared and analysed for choosing the most accurate one to carry on system evaluation. By combining normalized value of economic and efficiency index, the weighting factor based metric is used to compare the conventional AC distribution and DC distribution. Based on the time sequential simulation, the results show that under such novel optimization strategy, the energy efficiency is increased by 3.83% while the grid electricity saving is about 75.53 MWh accounting for 5% of the total annual load consumption.