This study evaluates the effects of electrical and optical parameters of building integrated photovoltaic (BIPV) windows with a semi-transparent solar cell on the overall energy performance of a typical mid-sized commercial building in various climate conditions. For precise evaluation of thermo-optical performance of the BIPV windows, we have fabricated semi-transparent amorphous silicon (a-Si:H) solar cells in various conditions and evaluated optical properties of the cells in addition to the energy conversion efficiency. More importantly, we have taken optical parameters of the cells in different sun wavelength spectra on the front and back side of the cell because optical response at each wavelength affects differently on the energy conversion efficiency of the cell and thermal properties of BIPV windows. By using three different semi-transparent solar cell types, the annual whole building energy simulation was performed to evaluate the BIPV window performance in six different climate conditions. A significant change in the overall building energy consumption was observed by varying the thermo-optical characteristics of BIPV windows. Our results suggest it is required to customize the BIPV window characteristics with real optical data through sun wavelength spectra which is sensitively varying by the fabrication conditions of solar cells, in order to maximize building energy performance for different climate conditions at the building location. (C) 2014 Elsevier Ltd. All rights reserved.