In recent years, the popularity of micro-grids has been growing with the rapid growth of renewable resources. In addition, a significant part of the energy globally consumed could be attributed to the building sector. Thus, to develop the expandable micro-grid clusters (EMGCs), the authors investigate optimal facility planning in the building energy management system (BEMS), which is one of the important component of EMGCs. To optimize the total facility cost of BEMS, the hybrid optimization of multiple energy resources, i.e., HOMER, a simulation tool developed by the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), is applied. A BEMS is simulated using photovoltaic generation, battery storage, a converter, and a load. The authors vary the eight types of battery capacity and investigate the correlation among the whole system for optimizing the facility configuration by minimizing the facility's lifecycle cost. The simulations reveal the optimal facility configuration that minimizes long-term costs.