To assess the PV-potential on rooftops for decentralized electricity supply in Switzerland, the potential is compared with the following urban-size parameters: (a) urban population, (b) number of buildings, (c) the cumulative building ground-floor area, and (d) the cumulative street length. The coefficients of determination R2 between the PV potential and these factors are: (a) 0.85, (b) 0.86, (c) 0.95, and (d) 0.93. The resulting relations show that 1% increases in factors a-d are associated with the following increases in PV-potential: (a) 0.82%, (b) 0.93%, (c) 0.94%, and (d) 1%, indicating sublinear relations with a-c, but a linear relation with d.