Comparing designs for photovoltaic-green roofs: A year-long field study in a subtropical climate

Photovoltaic-green roof systems combine solar energy generation with environmental co-benefits, yet their long-term performance under real-world conditions remains underexplored. Here we quantify system performance using a year-long, large-scale field experiment conducted in Hong Kong's subtropical climate, examining the effects of panel tilt angles (12° and 22°), separation heights (0.6, 0.75, and 0.9 m), and vegetation types (Wedelia trilobata, Sedum, and Zoysia). Compared with conventional rooftop photovoltaic setups, the best-performing photovoltaic–green roof configuration increased annual noontime energy yield by up to 3.7% while reducing peak under-panel air temperatures by 4.6 °C; however, not all photovoltaic–green roof configurations outperformed conventional photovoltaic systems. Among the tested designs, Sedum at 22° tilt and 0.6 m separation height achieved the highest annual energy yield, 6.7% higher than that of the lowest-performing photovoltaic–green roof configuration. In comparison with prior studies, our results suggest that annual energy gains are generally modest (typically below ∼3% across climate zones). The value of long-term, real-scale evidence lies in identifying when vegetation-driven cooling translates into electricity gains and when canopy growth and restricted under-panel airflow offset this benefit. Beyond electricity generation, photovoltaic–green roof systems can also provide rooftop co-benefits such as biodiversity support and urban agriculture, and our findings offer practical, performance-oriented design guidance for deployment.