Sustainable energy resources such as solar energy are in the spotlight to achieve post-carbon-society. However, the vast space is required to install PV generators compared with other distributed generators. The optimal usage of PV generator has to be investigated if it is installed in the limited areas. On the other hand, the output of PV power generator is not constant and varies depending on climate conditions. It causes difficulty in the power system operation and control when introduced into distributed networks or demand side. BESS (battery energy storage system) attracts attention as the device which is able to mitigate the change of PV generation output. As the first step in this paper, the relation between the solar radiation on PV and typical installation parameters such as azimuth and tilt angle of PV array is investigated as the usage of PV generators. In the second step, the combination of PV generation system and BESS is discussed from the viewpoint of cost and CO2-emission when installed on demand side, and the optimal installation capacities of PV generators and BESS are calculated.