The time-dependent geographical diversity of electric power output is essential to provide more rational basis for the promotion of photovoltaic (PV) power generation systems diffusion on a large scale. A set of regression formulae was developed to evaluate hourly global irradiance at about eight hundred sites throughout Japan. And regional distribution of hourly AC electricity output from very many PV stations were estimated, assuming that grid-connected 3 kW PV power generation systems are installed on all the roofs of detached houses in principal towns. The smoothing effect of PV output power fluctuation was examined in conjunction with interconnection capacity between power grids.