Rooftop solar with EV batteries could meet majority of Japan power demand

With limited land and growing demand for clean energy, Japan faces unique constraints. Yet, its vast rooftop area-over 8,000 km2-and a fast-expanding EV fleet offer untapped potential. The integrated system, termed "PV + EV" or "SolarEV City," offers both environmental and economic advantages, enabling municipalities to become energy self-reliant.

Researchers conducted a detailed techno-economic analysis of all 1,741 municipalities in Japan. Assuming 70% rooftop coverage with 20% efficiency solar panels and EVs equipped with 40 kWh batteries (half used for grid services), the team found rooftop PV alone could produce 1,017 TWh annually-exceeding Japan's 2022 electricity output. On average, rooftop solar could supply 45% of local electricity, but integrating EV batteries raised this to 85%. Additionally, the system could slash energy costs by 33% by 2030.

The study found strong potential in both rural and urban areas. Rural regions could generate far more electricity than they consume, while cities like Tokyo, despite spatial limits, could enhance grid stability using EV battery integration. The system also improves peak demand management and reduces reliance on fossil fuels.

"To make this system a reality and move towards a greener society, we need policy support at the end of the day," said Takuro Kobayashi of Tohoku University. The research aims to inform policymaking by highlighting regional potentials and challenges.

Key policy considerations include boosting support for vehicle-to-home (V2H) and vehicle-to-grid (V2G) technologies, addressing energy equity in low-solar regions, and increasing public awareness. Japan's current subsidies for EVs and rooftop PVs are a foundation, but further investment is critical to enable full system integration.

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