This study evaluated the energy performance of an a-Si semi-transparent PV insulating glass unit (IGU) via numerical simulation and experimental tests. Combined with the measured optical and electrical characteristics of the PV laminate, an integrated model was developed to simulate the overall energy performance of the IN IGU in EnergyPlus. Outdoor experiments were conducted to validate the accuracy of the simulation model. The proper consistency between the simulation results and the experimental data indicated that the simulation model was reliable for predicting the overall energy performance of PV IGU. With the validated model, a sensitivity analysis was conducted to investigate the influences of air gap depth and rear side glass, and an optimal design scheme was proposed to maximize the energy efficiency of PV IGU in Hong Kong. Finally, the energy saving potential of the optimized PV IGU compared to the other commonly used window systems was evaluated. It was found that the energy saving potential of the optimized PV IGU was 25.3% and 10.7%, respectively, compared to the single clear glass window and the Low-E glass window. The methodology developed in this study is expected to provide a useful reference for simulating the energy performance of PV IGU. (C) 2016 Elsevier Ltd. All rights reserved.