Damp heat and UVID show highest ‘red flag’ rates in 2026 RETC module testing report

Alongside the damp heat test failures, 8% of modules tested returned a red flag result in ultraviolent-induced degradation (UVID), the second-highest percentage among the types of tests. The PVMI report year-on-year increases in red flag results for damp heat, potential-induced degradation, static and dynamic mechanical load and thermal cycling testing categories; the results from these testing types, plus some selected others, are shown in the graph below.

It is worth noting that two of the above categories—conversion efficiency and hail durability testing—are unable to provide red flag results; RETC said it has “not applied red-flag criteria” for the former category, and that red flag criteria are not applied to hail testing because this is an optional testing sequence within the thresher test programme.

However, this should not be interpreted as these categories having strong testing performances. Conversion efficiency only returned 50% of high achievers, while hail testing yielded just 27% high achievers, the lowest among technology types.

Indeed, RETC noted that, since 2025, the percentage of high achievers in the hail testing category collapsed from 70%, suggesting that a significant number of module manufacturers struggle to build hail-resistant modules. This reflects growing concern about the ability of solar projects to effectively endure hail damage, with kWh Analytics, Kiwa PVEL and RETC’s parent company VDE Americas all highlighting the dangers of hail to solar project performance this year.

‘Underperformance appears to be increasing over time’

The report notes that, across all testing types, “underperformance appears to be increasing over time”. The RETC team analysed close to 44,000 system-months of data based on module testing, and assigned each module a numerical value between 0.6 and 1.2 to reflect quality of module performance. Between 2020 and 2024 the median score of modules tested, across all testing types, fell from 1.02 to 0.986; similarly, the average score of modules on a P90 basis fell from 0.96 to 0.85 over this period.

“At VDE, we are seeing an increase in certain types of field failures, including issues associated with junction boxes and PV connectors,” said Beth Copanas, senior director of technical advisory at VDE Americas. Copanas argued that “design optimisation efforts”, which aim to reduce the cost of manufacturing solar panels amid the ongoing price crisis affecting many Chinese manufacturers, could contribute to the decline in module performance.

“While these observations can have multiple contributing factors, they raise concerns that ongoing cost and design optimisation efforts may be affecting long-term durability. We are also seeing changes in module construction—such as reduced material thicknesses and tighter component tolerances— that may be contributing to increased sensitivity to field conditions.”

The report also concludes that market conditions are exacerbating some of the technical failures demonstrated by module testing. Cherif Kedir, CEO and president of RETC, argued that the “proliferation” of UVID can be traced to the popularity of tunnel oxide passivated contact (TOPCon) technology in the industry.

As China-based companies seek to relocate their TOPCon manufacturing capacity to parts of the world not covered by legislation that impedes the import of products to the US, most notably the Foreign Entity of Concern (FEOC) guidance and the ongoing antidumping and countervailing duty (AD/CVD) investigations, these “highly refined manufacturing processes” move outside of China, where there is considerable expertise in these processes.

“There are steps in the manufacturing process that dictate a cell’s ability to withstand some of these light-induced degradation modes,” said Kedir. “Manufacturers need to tightly control these steps. But if you take a highly refined manufacturing process out of China and move it to Africa, Eastern Europe, or the US, plant operators need to substantially revise the process definition.

“Without adequate batch testing, PV modules from a new factory could have a very different reliability profile, even though companies are nominally using the same manufacturing equipment and an identical BOM [bill of materials].”

RETC awards 13 ‘overall highest achiever’ status

However, the report is not without positives. Backsheet ultraviolet durability testing returned 100% of tests in the high achievement category, while light- and elevated temperature-induced degradation returned a high achiever percentage of 66%, the two highest figures for testing types that returned no red flags.

RETC also awarded a total of 13 companies its “overall highest achiever” status—Imperial Star, JA Solar, JinkoSolar, LONGi, Qcells, Runergy, Solarspace, Thornova, TrinaSolar, TW Solar, VSUN, Waaree and Yingli Solar—up from 12 the previous year. Nine of these companies—JA Solar, Jinko Solar, LONGi Solar, Runergy, Solarspace, Trina Solar, VSUN Solar, Waaree and Yingli Solar—have also received this award for at least three consecutive years, suggesting some companies have been able to build robust modules over a longer period.