Urban rooftops for food and energy in China

Urban rooftop agriculture (RA) and photovoltaic power production (RPV) offer sustainable solutions for the food–energy nexus in cities but compete for limited rooftop space. Here we explore the potential benefits (productivity, economic and environmental) and allocation strategy of RA and RPV across 13 million buildings in 124 Chinese cities, considering urban characteristics and regional productivity. We found that RA yields superior economic benefits, while RPV excels in greenhouse gas emission reductions. Prioritizing either RA or RPV can only retain 0–29% of the above benefits brought by the other. However, allocating 61% of the flat rooftop area to RA and all the remaining (including pitched rooftops) to RPV would retain >50% of their potential, meeting 15% (mean, 0.5–99% across cities) of urban vegetable needs and 5% (0.5–27% across cities) of the electricity needs. While the productivity from RA and RPV have significant environmental and socioeconomic benefits, they require considerable water (up to 15% of urban residential water use) and materials (for example, totaling 13 kt silver). This study proposes that the optimal allocation of roof area for rooftop agriculture and photovoltaics is 61% of the flat rooftop area to the former and the rest for the latter. However, maintaining this productivity requires considerable water use and materials.