Research News

A new study led by Tohoku University has revealed that rooftop solar panels, when combined with electric vehicles (EVs) as batteries, could supply 85% of Japan's electricity demand and reduce carbon dioxide emissions by 87%. The research provides a promising pathway for Japan's local governments to achieve carbon neutrality by taking advantage of existing infrastructure - rooftops and vehicles - rather than relying solely on large-scale energy systems.

As the urgency of climate change accelerates, cities and municipalities around the world are exploring ways to decarbonize their energy systems to slow down these changes. Japan, with its mountainous terrain and limited available land for solar farms, faces unique challenges. However, the country has more than 8,000 km² of rooftops and a rapidly growing EV market. The combination of rooftop photovoltaic (PV) systems and EV batteries--referred to as the "PV + EV" system or "SolarEV City"--offers a practical and cost-effective solution to help transition to renewable energy and energy self-sufficiency.

The study aimed to answer three critical questions: What is the decarbonization potential of PV + EV systems across all municipalities in Japan? What regional factors influence their effectiveness? How can these findings inform national and local energy policy?

To answer these questions, the research team conducted a comprehensive techno-economic analysis for all 1,741 municipalities in Japan. They assumed that 70% of rooftops would be fitted with 20%-efficiency PV panels, and that EVs with 40 kWh batteries would serve as household storage, using about half their battery capacity for grid flexibility. Their findings were striking: rooftop PV could generate 1,017 TWh annually--more than Japan's total electricity generation in 2022. On average, a PV-only system could supply 45% of municipal electricity needs, while the PV + EV system boosted this figure to 85%. In addition to drastically reducing CO₂ emissions from both electricity and driving, the system could lower energy costs by 33% by 2030.

These results carry broad implications not only for Japan, but for countries worldwide that face similar constraints. Rooftop PV + EV systems are particularly promising for urban and rural regions where centralized renewable energy systems may not be feasible. In rural areas, the study found that some municipalities could generate several times their electricity demand through rooftop PV alone. While highly urbanized areas like Tokyo face greater limitations due to smaller rooftop areas and fewer vehicles, they still benefit from the integration of EV batteries to increase energy flexibility and reduce peak demand.

"To make this system a reality and move towards a greener society, we need policy support at the end of the day," remarks Takuro Kobayashi (Tohoku University). "A major goal of this study was to provide a wealth of scientific information that policymakers can refer to when making decisions about implementation."

When making these policies, there are many factors to consider. Policymakers must address regional disparities, especially for northern areas with lower solar potential, where energy poverty could be exacerbated during the transition period. Furthermore, while Japan currently offers subsidies for EVs and rooftop PV systems, stronger support is needed for bidirectional charging infrastructure (V2H and V2G), battery integration technologies, and public awareness. This research exploring the potential of PV + EV systems is a crucial step closer towards decarbonization.

The study was published in Applied Energy on May 15, 2025. The research was led by Tohoku University in collaboration with The University of Tokyo, the National Institute for Environmental Studies, Radboud University (Netherlands), and the Meteorological Research Institute. The work was supported by public research funding including from Japan's Ministry of the Environment and JSPS KAKENHI.

Publication Details:

Title: On the decarbonization potentials of rooftop PVs integrated with EVs as battery for all the municipalities of Japan
Authors: Nguyen Thi Quynh Trang, Koharu Okada, Yusei Sugiyama, Takahiro Yoshida, Yujiro Hirano, Peraphan Jittrapirom, Tosiyuki Nakaegawa, Takuro Kobashi
Journal: Applied Energy
DOI: https://doi.org/10.1016/j.apenergy.2025.126067

Press release in Japanese