This paper investigates the energy payback time (EPBT) and greenhouse-gas payback time (GPBT) of roof-top building-integrated photovoltaic (BIPV) systems in Hong Kong. Five representational solar cells, i.e. mono-crystalline, multi-crystalline, amorphous silicon, CdTe thin film and CIGS thin film, were chosen for the study. The total energy requirement during the life cycle was collected and calculated. The largest cumulated energy requirement was about 5535 MJ/m², and the least one was about 2641 MJ/m² for mono-crystalline and CdTe modules BIPV systems respectively. The hourly solar irradiance and ambient air temperature of Hong Kong from 1998 to 2007 were used as weather data input. In order to maximize the received annual solar radiation by the PV panel, a computer program was compiled to simulate the annual solar radiation and to find out the optimum inclined angle of solar panel. It was found that the average usable solar radiation during the ten years was about 1333 kWh/m²∙year. The optimum inclined angle, vary with the weather data, was around 22±1 degree with the installation of south facing. Accordingly, the annual power generation of BIPV systems with different solar cells was estimated. Owing to high conversion efficiency, the mono-crystalline BIPV system could generate the most power electricity of 126 kWh/m², and the power generation of amorphous silicon PV system was found to be 59.4 kWh/m², which is the least one. According to the total energy requirement and annual power generation, the EPBT and GPBT were calculated. The amorphous silicon roof-top BIPV system has the longest EPBT, about 3.9 years, but for the CdTe thin film BIPV system it’s only 2.3 years. The GPBT for different solar cells was also discussed, ranging from 2.0 year (CdTe) to 2.9 years (amorphous silicon).