A research group led by Switzerland's University of Applied Sciences (SUPSI) has carried out a long-term analysis of six south-facing, grid-connected PV systems installed in Switzerland in the late 1980s and early 1990s. The researchers found that the systems’ annual power loss rates averaged 0.16% to 0.24%, significantly lower than the 0.75% to 1% per year commonly reported in the literature.

The study examined four low-altitude rooftop systems located in Möhlin (310m-VR-AM55), Tiergarten East and West in Burgdorf (533m-VR-SM55(HO)), and Burgdorf Fink (552m-BA-SM55). These installations use ventilated or building-applied rooftop configurations. The analysis also included a mid-altitude utility-scale plant in Mont-Soleil (1270m-OR-SM55) and two high-altitude, facade-mounted systems in Birg (2677m-VF-AM55) and Jungfraujoch (3462m-VF-SM75).

All systems are equipped with either ARCO AM55 modules manufactured by US-based Arco Solar, which was the world’s largest PV manufacturer with just 1 MW capacity at the time, or Siemens SM55, SM55-HO, and SM75 modules. Siemens became Arco Solar’s largest shareholder in 1990. The modules have rated power outputs between 48 W and 55 W and consist of a glass front sheet, ethylene-vinyl acetate (EVA) encapsulant layers, monocrystalline silicon cells, and a polymer backsheet laminate.

The test setup included on-site monitoring of AC and DC power output, ambient and module temperatures, and plane-of-array irradiance measured using pyranometers. Based on site conditions, the researchers classified the installations into low-, mid-, and high-altitude climate zones.

“For benchmarking purposes, two Siemens SM55 modules have been stored in a controlled indoor environment at the Photovoltaic Laboratory of the Bern University of Applied Sciences since the start of the monitoring campaign,” the researchers said. They also applied the multi-annual year-on-year (multi-YoY) method to determine system-level performance loss rates (PLR).

The results show that PLRs across all systems range from -0.12% to -0.55% per year, with an average of -0.24% to -0.16% per year, well below typical degradation rates reported for both older and modern PV systems. The researchers also found that higher-altitude systems generally exhibit higher average performance ratios and lower degradation rates than comparable low-altitude installations, despite exposure to higher irradiance and ultraviolet radiation.

The study further revealed that modules of the same nominal type but with different internal designs show markedly different degradation behaviour. Standard SM55 modules exhibited recurring solder bond failures, leading to increased series resistance and reduced fill factor. By contrast, SM55-HO modules benefited from a modified backsheet design that provides higher internal reflectance and improved long-term stability.

Overall, the findings indicate that long-term degradation in early-generation PV modules is driven primarily by thermal stress, ventilation conditions, and material design, rather than altitude or irradiance alone. Modules installed in cooler, better-ventilated environments demonstrated particularly stable performance over multiple decades.

The test results were presented in the paper “Three decades, three climates: environmental and material impacts on the long-term reliability of photovoltaic modules,” published in EES Solar.

“The study identified the bill-of-material (BOM) as the most critical factor influencing PV module longevity,” they concluded. “Despite all modules belonging to the same product family, variations in encapsulant quality, filler materials, and manufacturing processes resulted in significant differences in degradation rates. Early-generation encapsulants without UV stabilisation showed accelerated ageing, while later module designs with optimised backsheets and improved production quality demonstrated outstanding long-term stability.”

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