Still generating power after three decades. A study reveals what determines the lifespan of solar panels

Six photovoltaic power plants in Switzerland have been operating for more than three decades, and most of their panels still retain over 80% of their original output. A new study shows that degradation may not be as rapid as is commonly assumed. At the same time, it points out that climate conditions alone do not explain everything. The quality of the materials used and the design of the photovoltaic module may be even more important.

Photovoltaic panels are typically sold today with a performance warranty of 25 to 30 years. However, the warranty period does not represent the expected service life of the equipment. Once the warranty expires, a panel does not automatically stop generating electricity. The key question is how quickly its output will decline and whether operating the power plant will remain economically viable.

This is precisely the question addressed by the international study Three decades, three climates: environmental and material impacts on the long-term reliability of photovoltaic modules. Researchers analysed six photovoltaic systems in Switzerland that were commissioned in the late 1980s and early 1990s. Some of them have now been operating for approximately 35 years.

The results show that, in high-quality modules, long-term performance losses can be significantly lower than often assumed. The average annual performance loss of the analysed systems was only 0.24% per year.

“Most modules retained more than 80% of their original rated power even after 30 to 35 years of operation,” the study states.

Panels lost an average of a quarter of a percent of output per year

The researchers evaluated long-term changes in performance using the performance loss rate (PLR), which represents the annual rate of decline in system output. They worked with more than 20 years of operating data and compared electricity generation during equivalent periods in different years. This approach is intended to minimise the impact of seasonal weather fluctuations and year-to-year differences in solar irradiation.

The average PLR for all six systems was –0.24 ± 0.16% per year. Individual systems recorded values ranging approximately from –0.55% to –0.12% per year. Even the worst-performing system therefore remained below 0.6% annual degradation.

“The average annual performance loss rate of the analysed photovoltaic systems was only –0.24 ± 0.16%,” the authors state. This is a significantly better result than that reported in a substantial portion of the scientific literature, where values of approximately 0.75% to 1% per year are often used.

For illustration, if output declined evenly by 0.24% per year, a panel would still produce approximately 93% of its original output after 30 years. However, such a simple calculation cannot be directly equated with the results of laboratory measurements performed on individual modules.

Degradation does not necessarily follow a linear pattern. System performance is also affected by other factors, including connection failures, soiling, shading and differences between individual panels.

Laboratory measurements also showed that most modules retained more than 80% of their rated power. Some modules, however, degraded much more significantly. The differences between individual module types and production batches proved to be one of the most important findings of the research.

Lower temperatures help, but they are not the only factor

The six systems were located in different parts of Switzerland and at different altitudes. This allowed the researchers to compare panel operation in warmer lowland locations with higher-altitude sites exposed to colder conditions.

Modules installed at high-altitude locations operated at temperatures up to 20°C lower than those installed in the lowlands. The average annual performance loss was approximately 0.11% at higher elevations, compared with about 0.35% per year at lowland sites.

The difference was not related to altitude alone. The mounting method and ventilation of the panels were also important. The highest temperatures were recorded in a building-integrated system, where module temperatures reached almost 80°C. This type of installation restricts airflow behind the panel and can increase thermal stress.

Higher operating temperatures affect photovoltaic panels in two ways. In the short term, they reduce their instantaneous electrical output. Over the long term, they can accelerate the ageing of the polymer materials used for encapsulation and in the rear section of the module. Repeated heating and cooling also create mechanical stress between materials with different coefficients of thermal expansion.

Laboratory analyses confirmed that the EVA encapsulant degraded more rapidly at higher temperatures. Its chemical decomposition produced acetic acid, which can contribute to corrosion of metal components and electrical connections. Some modules also showed signs of degradation in the adhesives used in the backsheet.

Materials mattered more than climate

Perhaps the most important finding of the study is that climate conditions alone do not explain the differences in degradation. The researchers identified the specific composition of the materials used in module manufacturing as the most significant factor.

“The bill of materials (BOM) is the most important factor in ensuring the long service life of photovoltaic modules,” the authors state.

All of the panels examined belonged to the same AM55 and SM55 product family. Despite this, their long-term behaviour differed considerably. The differences were associated with the quality of the EVA film, the fillers used, the design of the rear layer and the manufacturing processes.

Older modules manufactured before 1987 used EVA materials without sufficient protection against ultraviolet radiation. Over time, these modules showed yellowing and delamination between individual layers. Later modules manufactured in the early 1990s already used modified materials and achieved significantly better stability.

The results also showed that some modules sold under similar product names did not necessarily have the same internal construction. Some panels experienced failures in soldered joints, which increased series resistance and reduced the so-called fill factor. Other variants using modified backsheets demonstrated better long-term stability.

The researchers therefore warn that assessing service life requires more than simply knowing the manufacturer or commercial name of a module. The specific materials used and changes in the manufacturing process can also be decisive.

“Our results show that the quality of the materials used is the decisive factor in the long-term reliability of photovoltaic modules,” the study states.

Results are better than the average reported in previous studies

The average performance loss of 0.24% per year is significantly lower than the values reported in many earlier studies. A 2012 analysis by the U.S. National Renewable Energy Laboratory (NREL) compiled almost 2,000 measurements of photovoltaic module and system degradation. The median degradation rate was approximately 0.5% per year.

A more recent analysis published in 2025 examined 610 observations from 80 primary studies. It found an average degradation rate of 1.1% per year and a median of 0.94% per year. However, the authors also highlighted the considerable variability in the results. Cell technology, climate conditions, mounting method and measurement methodology all played important roles.

Direct comparison, however, has its limitations. The Swiss study assessed a small number of specific systems using similar modules, whereas meta-analyses combine results from different technologies, countries, climate zones and measurement methods. The indicators themselves also differ. Some studies evaluate the decline in rated module power under laboratory conditions, while others assess the decline in the output of an entire system during operation.

A new large-scale analysis of German photovoltaic systems, for example, used operational data from approximately 1.25 million systems with a combined capacity of 34 GW. It found an average performance decline of around 0.59% per year. The results also showed that larger systems degraded faster than smaller ones, while high temperatures, frost and air pollution all had a negative effect.

Old panels may not be representative of new ones

The findings of the Swiss study cannot automatically be applied to today’s photovoltaic panels. The modules examined were manufactured in the late 1980s and early 1990s and featured relatively robust construction. They used comparatively thick silicon cells, sturdy aluminium frames and durable multilayer backsheets.

Modern modules are more efficient, lighter and cheaper. Manufacturers use thinner silicon wafers, new cell technologies, different encapsulation materials and, increasingly, glass-glass designs. Some of these changes may improve service life, but the rapid pace of technological development also means that decades of real-world operating data are not yet available for many newer technologies.

The authors therefore caution that the historical durability of older modules does not automatically guarantee the same service life for today’s products. Modern panels may achieve even better results, but their long-term reliability can only be confirmed after decades of operation.

With low degradation, service life could reach 50 years

Based on the results, the authors suggest that high-quality photovoltaic modules operating in a temperate climate could achieve service lives exceeding 50 years. However, this is not a direct measurement result. It is an extrapolation based on the low degradation rates observed and on the fact that most of the analysed panels still retained high output after three decades.

Such an estimate should be understood as the potential of the best-performing systems rather than as a general service-life figure for photovoltaic panels. The outcome depends on manufacturing quality, climate conditions, mounting method, ventilation, maintenance and the specific failure mechanisms involved.

Nevertheless, the study provides an important practical insight. When assessing the economics of photovoltaic power plants, it should not automatically be assumed that panels will need to be replaced once their 25- or 30-year warranty expires. If the modules retain sufficient output and show no serious defects, they may continue generating electricity for many more years.

A longer service life would also reduce the cost of generated electricity, the amount of waste produced and the quantity of materials required to manufacture replacement panels. From an investor’s perspective, it could increase the lifetime electricity generation of a power plant and improve its overall economics.

Source: https://www.energie-portal.sk/