The efficiency of building-integrated photovoltaic(BIPV) system is mainly determined by solar radiation and the temperature of PV modules. The performance of BIPV systems is reported to be different from that of conventional PV systems installed in the open-air. This paper presents the relationship of solar radiation and electricity generation from a 2kWp roof-integrated PV system that is applied as building elements on an experimental house, and the energy saving effect of the BIPV system for a typical house. For the performance evaluation of the BIPV system, it produced a regression equation with measured data for winter days. The regression equation showed that a comparison of the measured electricity generation and the predicted electricity generation of the BIPV system were meaningful. It showed that an annual electricity generation of the system appeared to cover around 52% of an annual electricity consumption of a typical domestic house with the floor area of $96m^2$.