A research team in Japan has revealed a transformative pathway for the country’s energy future, showing that combining rooftop solar panels with electric vehicle (EV) batteries could meet up to 85 percent of the nation’s electricity needs while cutting carbon dioxide (CO₂) emissions by 87 percent.

The study, carried out by Tohoku University, along with the University of Tokyo, the National Institute for Environmental Studies, Radboud University in the Netherlands, and the Meteorological Research Institute, provides a detailed look into how existing infrastructure – rooftops and vehicles – can be harnessed for large-scale decarbonization.

The researchers claimed that with the growing urgency of climate change, Japan faces distinct challenges in decarbonizing its energy systems, largely due to its mountainous terrain and limited space for large-scale solar farms.

Yet, with over 3,000 square miles (8,000 square kilometers) of rooftop space and a rapidly expanding electric vehicle market, the researchers proposed combining rooftop photovoltaic (PV) systems with EV batteries in a model called the “PV + EV” system, or “SolarEV City.”

The team believes the approach could offer the Pacific country a practical and cost-effective pathway to renewable energy and greater energy self-sufficiency.

A new path to clean energy

To evaluate the concept’s potential, the team carried out a comprehensive techno-economic analysis of all 1,741 municipalities in Japan. They assumed 70 percent of the rooftops would be equipped with solar panels operating at 20 percent efficiency, while EVs with 40 kilowatt hours (kWh) batteries would provide household energy storage, using about half their battery capacity to support the grid.

Following their analysis, the researchers were stunned to discover that rooftop photovoltaics alone could generate 1,017 terawatt-hours (TWh) per year, surpassing the country’s total electricity production in 2022.

Electricity self-sufficiency by rooftop PV + EV.
Credit: Takuro Kobashi et al.

Meanwhile, they further discovered that a PV-only system could supply 45 percent of municipal electricity needs. When combined with EV battery storage, the figure significantly jumped to 85 percent.

In addition to lowering emissions from both energy generation and driving, the PV + EV setup could reduce energy costs by 33 percent by 2030, making it a financially attractive option for households and governments alike.

Challenges and solutions

The researchers highlighted that the findings have profound implications not only for Japan but also for countries worldwide facing similar challenges. Rooftop PV + EV systems offer a promising solution for both urban and rural areas where centralized renewable energy infrastructure may be difficult to implement.

While rural areas could even generate a surplus of electricity from rooftop PV alone, densely populated cities like Tokyo face greater challenges due to limited rooftop area and fewer vehicles. However, even in these urban environments, EV batteries can significantly increase energy flexibility and help 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,” Takuro Kobayashi, PhD, a chief senior researcher at Tohoku University, said in a press release.

“A major goal of this study was to provide a wealth of scientific information that policymakers can refer to when making decisions about implementation,” Kobayashi concluded.

The team believes policymakers must consider regional differences, especially in northern areas with lower solar potential, where energy poverty could worsen. While Japan currently supports EVs and rooftop solar with subsidies, more backing is needed for bidirectional charging infrastructure (V2H and V2G), battery integration technologies, and public engagement.

The study has been published in the journal Applied Energy.