How to avoid ‘long tail’ effects in large-scale PV plants

New UNSW research found that about 20% of solar modules in large PV plants degrade much faster than expected. They recommend holistic strategies such as robust materials, advanced designs, and proactive monitoring to decouple degradation pathways and prevent cascading failures.

January 9, 2026
Emiliano Bellini

A group of researchers from University of New South Wales (UNSW) studied the “long tail” phenomenon in large-scale photovoltaic plants and found that about 20% of the solar modules used on the field degrade much faster than expected.

The “long tail” occurs when a significant number of modules in the same facility underperform relative to expectations, creating a substantial risk for asset owners. It’s called “long tail” because the graph showing the energy output over time presents a curve with a high peak and a long, low tail.

“The research shows that the long-tail of extreme PV degradation is an intrinsic feature of PV fleets and is observed across all climates and project types, rather than being limited to specific regions or technologies. Even when data are analysed within individual climate zones such as hot desert, Mediterranean, temperate, a pronounced long tail persists, indicating that extreme underperformance is not simply a result of aggregating different climates,” the research’s corresponding author, Shukla Poddar, told pv magazine. “That said, climate and project context influence how severe the tail becomes. Hot, dry or hot–humid regions tend to exhibit higher average degradation rates and heavier tails because thermal stress, UV exposure, and moisture accelerate multiple degradation mechanisms simultaneously.”

“The study also shows that the most extreme degradation often occurs when multiple mechanisms co-occur and reinforce each other, for example, backsheet degradation enabling moisture ingress, which then accelerates corrosion, hot spots, and discoloration,” she went on to say.  “Avoiding this interplay requires robust material selection such as durable backsheets or glass–glass designs, compatible encapsulants, module and architectures that limit electrical mismatch, rigorous manufacturing quality control to reduce infant mortality, and proactive monitoring and maintenance in the field. Standards bodies are moving toward combined-stress testing for simultaneous UV/heat/humidity cycling, so modules survive real-world conditions. In practice, this means factories should not only meet each IEC test individually but also consider synergistic loads. Collectively, these measures aim to decouple degradation pathways, so that an initial defect or stressor does not propagate into a chain of failures that pushes modules into the extreme “long tail” of degradation.”

The main factors contributing to the “long tail” are, among others, potential-induced degradation (PID, light-induced degradation (LID), thermal cycling or temperature stress, humidity and moisture ingress, mechanical stress caused by wind, snow, and hail, as well as UV-induced polymer degradation in encapsulants and backsheets.

To investigate them, the researchers proceeded in three main steps. First, they examined whether the long tail is simply a statistical artifact resulting from aggregating global data, by conducting separate analyses for each climate zone. The, they explored the relationships among eight common degradation mechanisms to determine whether their co-occurrence is a defining feature of severely degraded modules. Finally, they considered the temporal dynamics of the long tail, analyzing both the age distribution across the full dataset and longitudinal evidence from individual systems to understand module wear-out over time.

The analysis showed that long tail observed in field data reflects a combination of three factors: early failures from initial defects, performance decline due to interacting degradation mechanisms, and long-term wear-out of latent defects. This multifactorial view explains, the researcherds said, why real-world degradation distributions are “skewed.”

“The long tail appears on graphs showing the degradation rate per year of the panels, indicating that up to 20% of all samples perform 1.5 times worse than the average,” they also stated. “In other words, a significant number of panels do not degrade at a constant rate over a long period of time as might be anticipated, but instead lose energy or fail unexpectedly much sooner.”

They suggested to address all issues related to the “long tail” phenomenon through a holistic approach. “In the context of our research, a holistic approach means moving beyond treating individual failure modes in isolation and instead designing, manufacturing, and operating PV systems to prevent cascading or interacting degradation mechanisms,” Poddar explained. 

As main mitigating strategies, the group proposed decoupling degradation pathways through robust design and reducing degradation interplay through advance module structure. “Future work and industry efforts should focus on improving initial quality, understanding and preventing the interaction between associated degradation modes, and implementing module designs that are inherently more tolerant to the inevitable emergence of cell-level inconsistencies over time,” they concluded.

Their findings can be found in the study “Understanding and Reducing the Risk of Extreme Photovoltaic Degradation,” published in the IEEE Journal of Photovoltaics.

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