The rise of mixed-use buildings contributes to the sustainable development of cities but are still met with challenges in energy management due to the lack of energy efficiency and sustainability guidelines. The use of integrated renewable-storage energy systems is a more beneficial solution to this problem over individual solutions; however, most design studies in the literature only focus on single-type of buildings or solutions. Thus, this study aims to develop an optimization model to determine the design capacities of an integrated energy system for mixed-use buildings. The objective is to minimize net present costs, subject to sizing, energy balances, and operational constraints. The case study considered a mixed-use building in a tropical area, with solar photovoltaic system as the renewable energy source and Lithium-ion battery as the energy storage system technology. The model was developed in HOMER Grid software, and economic metrics were used to evaluate and compare the proposed system to the following design cases: business-as-usual, stand-alone renewable source, and standalone energy storage. The results show that the integrated system has the lowest equivalent annual costs despite its high capital costs. Still, it has the most attractive economic case based on the evaluation metrics compared to other design cases.