On the decarbonization potentials of rooftop PVs integrated with EVs as battery for all the municipalities of Japan

Rooftop photovoltaics (PV) systems are often underestimated in their decarbonization potential due to technical, economic, and social barriers. However, rapid advances in PV technology indicate that rooftop PV could play substantially more roles than generally thought, particularly by coupling with EVs as batteries (“PV + EV”). We conducted a techno-economic analysis for rooftop PV integrated with EVs (Battery Electric Vehicles: BEV with 40 kWh battery and about half of the capacity is used as flexibility) for all the 1741 municipalities in Japan. Assuming 70 % of rooftop areas are covered with PV systems with 20 % efficiency, we estimate Japan's total rooftop PV capacity as 1155 GW generating 1017 TWh of electricity. It is 1.2 times larger than Japan's total electricity generation (834.8 TWh in Fiscal Year 2022). The “PV only” system can supply 45 ± 4 % of the hourly electricity demands of all the municipalities on average. By integrating with EVs, the “PV + EV” system can supply 85 ± 12 % of the demands, reducing carbon emissions by 87 ± 11 % from electricity generation and driving, while potentially saving costs by 33 ± 11 %. We also found that “PV + EV” has limited roles in highly urbanized areas such as Tokyo special districts owing to relatively small rooftop areas, but in rural areas, it could supply up to 98 % of electricity demands with multiple times more electricity generated than their demands. We observe significant regional variation in decarbonization potential between northern and southern Japan, driven by differences in total insolation, its seasonal and diurnal distribution, and electricity demand characteristics. The north-south differences may impact energy poverty requiring policy attention under the government's increasing decarbonization policy. At present, the market for Vehicle to Home (V2H)/Vehicle to Grid (V2G) is generally limited in Japan. To unleash the potential of rooftop PV systems integrated with EVs as batteries for rapid urban decarbonization, more policy attention is urgently needed, providing supports for the infrastructure, R&D, and demonstrations of the “PV + EV” systems around the world.