Abstract This study develops an approach for optimizing the size/scale of microgrids used in electricity sharing around each residence by considering the uncertainty between the electricity supply from photovoltaics and electricity demand. Uncertainties are quantified using simulations that consider actual daily variations in supply and demand. The developed approach is applied to microgrid optimization in Sumida-ku, Tokyo, Japan, and results suggests that large-scale microgrids are required in central districts but small microgrids are sufficient in old residential areas. Results also show the statistically significant effect of optimizing microgrids to enhance energy self-sufficiency.