Around a fifth of the solar panels examined in a news study by the University of New South Wales (UNSW) were found to fail much faster than expected, with some likely to last for only half of their lifetime.

Researchers identified the reason behind the so-called ‘long tail’ in the probability distribution of the performance data after analyzing information obtained from nearly 11,000 different photovoltaic samples globally.

When the researchers plotted yearly degradation rates from a large global dataset, they observed a long tail of outliers in which systems performed far worse than expected.

According to the report, the typical decline across the dataset is around 0.9% per year, and nearly 20% of all samples perform 1.5 times worse than the average.

For some, the useful life is around 11 years, and they could lose nearly 45% of their output over a 25-year lifespan.

The report attributed the ‘long tail’ bad performers mainly to the cascading effect, which means that one defect triggers other failures. Also, some modules fail early due to manufacturing defects that slip past quality control.

The researchers found that there are interconnected failures, when different types of failures can interact with each other on an individual panel. The domino effect, when issues don’t just add up but multiply, can be seen as making modules degrade faster than expected.

Also, there are minor initial flaws that may not cause problems at first but can lead to severe performance degradation over time.

The researchers added that climate alone doesn’t explain the ‘long tail,’ as degradation patterns persist even when very hot climate regions are excluded.

Shukla Poddar, a co-author of the paper, said: “A subset of the data shows information specifically related to solar modules in very hot climates, which we know causes higher degradation.”

“However, in other climates, when those hot regions are being excluded from the analysis, we see a similar long-tail pattern in the probability distribution of performance degradation rate. This suggests that the issue is consistent regardless of where the panels are operating.”

The researchers added that even if 20% are bad outliers, a utility-scale project has thousands of modules, and the ‘long tail’ becomes a major financial and energy-yield risk.

The report adds that current tests are not enough, as real-world operations involve many interacting features, so the industry needs better testing standards and more real-world data from large farms to reduce these surprise failures.

In 2025, the Fraunhofer Institute for Solar Energy Systems ISE found that ultraviolet tests rate degradation of TOPCon solar modules significantly higher than performance reduction in real-life conditions. The research shows that common UV tests can significantly exaggerate the degradation effect.

Earlier this year, researchers from UNSW and the Chinese solar module manufacturer LONGi found that the rear side of TOPCon cells, particularly the silicon nitride layer, is prone to chemical degradation when exposed to sodium-based salts, leading to a significant loss of open-circuit voltage.