This article is based on analysis published by SolarQuotes
A group of solar photovoltaic (PV) modules installed in Switzerland in the late 1980s and early 1990s has delivered a rare, long-term insight into how well solar panels really age. The results suggest today’s performance warranty assumptions are not only plausible, but conservative.
Researchers from Switzerland, Austria and Germany have analysed the long-term performance of six PV systems operating for up to 30 years, supported by more than two decades of high-quality monitoring data. Their findings indicate that well-manufactured modules from that era have comfortably remained within the degradation rates typically assumed in modern performance warranties.
The study offers timely perspective as manufacturers increasingly offer 25-30 year product and performance warranties, and as asset owners place greater emphasis on long-term yield, reliability and levelised cost of energy (LCOE).
Product versus performance warranty
In today’s market, most tier-one manufacturers offer product warranties of 25 to 30 years, covering defects in materials and workmanship. This sits alongside a performance warranty, typically for a term of 25 to 30 years, which guarantees that a module will retain a defined percentage of its original nameplate power output over time, subject to a maximum annual degradation rate.
By way of example, many contemporary modules guarantee around 88-92 per cent of original output after 25 years, with allowable degradation typically in the range of 0.35 to 0.5 per cent per year from year two onwards.
Historically, product warranties were much shorter than performance warranties, and the long-term performance claims were often treated with scepticism by parts of the industry. This study provides rare empirical evidence over three decades to test those assumptions.
Modules and test conditions
All PV installations in the study used modules from the same product family: ARCO AM55, Siemens SM55 (including the high-output SM55-HO variant), and Siemens SM75. All used aluminium back-surface field (Al-BSF) cells.
The modules were manufactured and installed between 1987 and 1993. Most had a nominal power rating of 55 W (48 W for the SM75), with an open-circuit voltage of 21.7 Volts (V) and a short-circuit current of 3.45 A.
In addition to the field-installed systems, two Siemens SM55 modules were stored indoors in a controlled environment at the Photovoltaic Laboratory of the Bern University of Applied Sciences. These unexposed reference modules allowed researchers to isolate material ageing effects from outdoor environmental stress.
The systems were installed across three climate zones, categorised by altitude: low, mid and high altitude. Altitude was chosen as a key variable due to its strong influence on temperature, irradiance and weather patterns in Switzerland. The sites also corresponded to different Köppen-Geiger climate classifications.
Key findings
Across all systems, the average performance degradation rate was measured at just −0.24 ± 0.16 per cent per year. This is comfortably within, and in many cases better than, the degradation rates assumed in modern warranties.
Laboratory measurements confirmed that most modules retained more than 80 per cent of their original nominal power after 30 to 35 years of operation. This aligns well with today’s typical performance guarantees, which often promise 80-90 per cent output after 25 to 30 years.
One notable finding was the impact of operating temperature. Systems located at lower altitudes, where ambient temperatures were up to 20°C higher, experienced greater thermal stress. This led to accelerated encapsulant degradation and acetic acid formation, causing localised corrosion and higher performance losses compared to cooler, higher-altitude sites.
What this means for modern modules
The researchers note that these results highlight the durability of early 1990s module designs featuring EVA encapsulants, Tedlar backsheets, and robust glass and frame constructions. The modules in the study used front glass around 3.3 millimetres (mm) thick, significantly thicker than some modern modules which may use glass as thin as 2.0 mm or even 1.6 mm.
While EVA encapsulants remain widely used today, polyolefin (PO) materials are increasingly gaining market share, particularly in TOPCon bifacial modules with glass-glass construction. Tedlar backsheets also continue to be widely used and are still regarded as a benchmark for long-term durability.
However, the researchers caution that many degradation mechanisms affecting today’s advanced cell architectures (such as PERC, TOPCon and SHJ) were not present in the Al-BSF cells used in the older modules. This means direct performance extrapolation should be approached carefully.
Beyond the fine print
The study also underscores a reality well known to the industry: Warranties are only as valuable as the companies that stand behind them. Several manufacturers from the early PV era, including Siemens, have long since exited module manufacturing.
Installation quality remains another decisive factor. Poor installation practices can significantly accelerate degradation, regardless of module quality.
Nevertheless, the researchers conclude that long-term field studies of this kind remain essential for guiding improvements in module design, manufacturing standards and system durability.
“Ultimately, well-designed modules and systems have the potential to operate well beyond conventional warranty periods, contributing to lower LCOE, reduced carbon footprints, and extended service lifetimes for PV systems,” the authors noted.