This paper extends the concept of our proposed - electricity sharing system as a complement or alternative to a feed-in tariff (FiT) to achieve CO2-neutral transportation in cities. In our proposed system, electricity generated from widely introduced solar photovoltaic panels (PVs) is stored in the 'cars not in use' in a city. In Japan, almost half of the cars in the central Tokyo metropolitan area are used only on weekends and thus are kept parked during weekdays. These cars represent a huge new potential storage depot if they were replaced by electric vehicles (EVs), that is, they could be used as storage batteries in a V2G system. The results of our previous study  showed that although the entire electricity surplus (PV supply minus demand) could be stored without waste if 12% of the EVs not in use were utilized as storage batteries at an aggregate (city) level in August (with maximum solar irradiance), there exist significant regional mismatches at the local district level. Hence, based on the geographical electricity surplus estimates, this paper develops a metaheuristic-based spatial clustering algorithm to find optimal spatial clusters where local mismatches between electricity surplus and storage potential is minimized, and self-sufficient green electricity is achieved.