Solar Panel Problems and Degradation explained

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Solar panels are generally very reliable and trouble-free as they have no moving parts and require minimal maintenance other than cleaning. However, like any manufactured product, solar panels can fail or underperform due to faulty materials or poor workmanship during the manufacturing process. Fortunately, this is very rare, and usually, only 1 in 5,000 panels will suffer from a manufacturing defect. Defects are often associated with the constant drive to reduce costs, and not surprisingly, this is why lower-cost panels generally suffer more faults compared to panels from well-established premium solar brands.

Also, see our detailed Solar System Fault Finding Guide

In addition to the small number of manufacturing defects, it is normal for solar photovoltaic (PV) cells to experience a small amount of degradation over time. Solar panels must operate for many years in a wide variety of extreme environments, from climates with huge temperature fluctuations to high humidity, rain, storms, strong winds, and corrosion from salt in coastal areas. Despite this, almost all solar panels have a minimum 10-year manufacturer warranty and are also backed by a 25-year performance warranty—learn more about solar panel warranties.

Common Solar Panel Problems

Over the expected 25-year life of a solar system, it is normal for the performance to slowly reduce over time, but unfortunately, one or more panels may fail at some point due to the five well-known phenomena listed below. In addition to these reasons, solar panels can sometimes be damaged during transportation or mishandled during installation, which may not become apparent until several years after the installation. Also, in rare cases, the front glass can be shattered due to severe impacts from very large hail and other projectiles. Note, of the five reasons listed below, the first is not technically a defect but a very slow loss in performance over the life of the solar panel.

Six reasons for solar panel degradation and failure:

1. LID - Light Induced Degradation - Normal performance loss of 0.25% to 0.7% per year

2. PID - Potential Induced Degradation - Potential long-term failure due to voltage leakage

3. General Degradation - Premature failure due to water ingress or other defects

4. LeTID - Light and elevated Temperature Induced Degradation - sudden 3% to 6% loss in performance

5. Micro-cracks and hot spots - Longer-term defects and failure due to broken or damaged cells

6. Failed bypass diodes - A defect often related to solar panel shading from nearby objects.

1. LID - Light Induced Degradation

When a solar panel is first exposed to sunlight, a phenomenon called ‘power stabilisation’ occurs due to traces of oxygen in the silicon wafer. This effect has been well studied and is the initial stabilisation phase of light-induced degradation (LID). During this phase, it is normal for a solar panel to lose 2% to 3% of its rated wattage (Wp) output in the first few hundred hours of operation, and the full effect of this initial phase occurs during the first year of use.

After the initial stabilisation phase, the rate of LID reduces significantly, reducing to 0.3% to 0.6% per year for the next 25+ years. However, LID can be as low as 0.25% per year on high-performance modules from manufacturers like Sunpower and REC, thanks to the high-purity N-type silicon cell substrate used. Fortunately, most manufacturers slightly over-spec the panel power rating by up to 5%, which takes into account slight cell imbalances and offsets most of the initial degradation. This also ensures the rated panel power (Wp) is accurate. For example, a 350 Watt panel may initially produce up to 5% more power or up to 368 Watts for a short amount of time. However, this slight overproduction is generally short-lived and may not be measurable unless the panels are operating under the ideal (STC) conditions. The manufacturer’s performance warranty will describe the rate of LID and the expected performance loss over the (25-year) warranty period.

2. PID - Potential Induced Degradation

Potential-induced degradation, or PID, is a form of panel power degradation that can become apparent after 5 to 10 years of use due to high voltage, elevated temperatures, and high humidity. This does not happen on all panels, especially those in less humid climates, but it has been found to occur on various first-generation Monocrystalline PERC cell panels produced from around 2016 to 2020. PID is essentially a voltage leak from the cells to the frame of the solar panel resulting in reduced power output. Unfortunately, the problem may not be initially noticeable, but over time, it usually becomes progressively worse, resulting in up to 20% or more power loss. PID can be difficult to diagnose without a specialised IV curve tester and training. However, an early indication can be an abnormally low string voltage or current. Find more information about diagnosing problems in our solar system fault-finding guide.

Most residential rooftop solar arrays operate in the 300 to 600-volt range, and PID is more prominent with higher string voltages; therefore, the more panels connected in a string, the greater the chance of PID occurring. Large-scale solar farms often operate in the 1000 to 1500-volt range, so the chance of PID is much higher. Fortunately, there are some advanced large-scale solar inverters that can reverse the effect of PID, if detected, by running a very small reverse current overnight. In very serious cases where PID issues were not addressed after 10 or more years, the power output can be severe, with up to 50% power loss. Fortunately, many leading solar panel manufacturers have almost eliminated the risk of PID by using high-quality materials and undertaking rigorous testing. However, it’s still an ongoing problem, as highlighted by the latest test results from independent testing institution PVEL.

3. General degradation

In addition to the well-known PID and LID effects, panels can also suffer from more serious issues due to the breakdown of the encapsulant and protective layers that are supposed to protect the cells from the elements. The most common of these is back-sheet failure. While the front glass sheet protects the solar cells from rain, hail, dirt and debris, the white or black plastic back-sheet is designed to protect the rear side of the cells from water, humidity and scuffs. However, often due to substandard material selection and poor quality control, UV radiation can cause either the encapsulant or rear protective back-sheet to break down, crack or degrade over time. This degradation can then lead to more serious issues such as moisture ingress, corrosion and earth leakage.

Published Research highlighting increased degradation in older panels

For an in-depth analysis of the potential faults and observed degradation rates in older polysilicon solar panels, see the detailed research paper titled ‘Degradation analysis of polycrystalline silicon modules from different manufacturers under the same climatic conditions”. Note that although polysilicon solar panels are no longer manufactured, thousands of polysilicon solar systems installed worldwide may need attention due to the accelerated degradation of one or more panels.

4. LeTID

Most modern silicon crystalline solar panels contain PERC solar cell technology, which increases panel efficiency and has been adopted by the majority of the world’s solar panel manufacturers. However, it has only recently become apparent that P-type PERC cells can suffer what is known as LeTID, or light and elevated temperature-induced degradation.

The LeTID phenomenon is similar to LID, although the losses due to LeTID have been recorded to be as high as 6% in the first year and, if not fully accounted for by the manufacturer, could lead to poor performance and potential warranty claims. Fortunately, N-type silicon cells from several manufacturers, including Sunpower and REC, do not suffer the effects of LeTID. Also, several manufacturers who use P-type poly and mono PERC have developed processes during manufacture to reduce or eliminate any LeTID losses, this includes Q Cells who were the first to claim anti-LeTID technology on all panels.

Many of the world's leading manufacturers, including Jinko Solar, Trina Solar, Longi Solar, and GCL, recently gained certification against LeTID from TÜV Rheinland. Others who also claim to have reduced or accounted for the effect of LeTID include REC, Winaico, and Canadian Solar. See the latest updates and information on manufacturers meeting the challenges of LeTID in the detailed articles from PV Tech.

5. Micro-cracks and hot spots

Most modern solar panels are made using a series of solar cells made from ultra-thin crystalline silicon wafers. The wafers are typically around 0.16mm thick or about twice the width of a human hair. Naturally, the wafers and cells are quite brittle and can crack or fracture under high mechanical stresses like mishandling during installation, extreme wind loads or large hail. It’s worth noting that not all cells are brittle; the high-performance IBC cells used by Sunpower are much stronger due to the large array of back-contacts that reinforce the cell.

Any unusual loads or stresses, such as people walking on solar panels during installation or maintenance, can lead to micro-cracks, which can create hot spots over time and eventually lead to panel failure. Micro-cracks can also form during transportation, impacts, dropping or rough handling.

Micro-cracks

Micro-cracks can be hard to detect and are often invisible at first. Tiny fractures in solar cells are often visible on older panels and will appear as snail trails on the surface of the cell. These fractures do not always cause a significant problem, and the panel may still perform well for many years, even with several fractured cells. However, micro-cracks will start to become a more serious issue if they increase the internal resistance and break the flow of current, leading to a hot spot or hot cell. This is particularly problematic if a micro-crack is large or forms across the entire cell. Fortunately, most modern solar panels now feature half-cut cells with multi-busbars, which significantly reduces the detrimental effects of microcracks. Also, shingled cell panels such as those from Hyundai and Sunpower are generally not susceptible to micro-cracks due to the unique overlapping configuration.

Hot spots

Solar cells generate an electrical current that flows through the interconnected cells. If this is compromised due to an internal fault or severe micro-cracking, then the increased resistance generates heat, which in turn increases the resistance further and creates more heat, resulting in a hot spot. In severe situations, this can even burn the cell. For more information, this detailed article from PV magazine explains the mechanisms behind micro-cracking and how new panel designs and innovations can help reduce the likelihood of micro-cracks forming.

Hot spots and micro-cracks are not always visible to the naked eye, and often, the only way to determine if a solar panel is compromised is to use a specialised thermal imaging camera that will highlight the temperature difference between the various cells. It’s worth noting that regular shading from rooftop obstacles can in some cases cause hot spots to form over several years due to the reverse current effect of the shaded cells.

6. Failed bypass diodes

When bypass diodes in solar panels are activated due to severe shading, they can dissipate some of the electrical energy as heat. This can lead to overheating if the diodes operate continuously under shaded conditions, increasing the risk of failure and often leading to hot spot formation and panel failure. Factors such as high temperatures, humidity, and electrical stress from continuous operation exacerbate the likelihood of diode failure. The extent of shading—whether light or heavy—affects the activation frequency and stress level on the diodes. Light shading, such as that caused by nearby trees, generally poses less of a problem than heavy shading from objects like closely positioned rooftop installations.

Diagnosing a failed bypass diode can vary in difficulty; it's relatively straightforward in older solar panels where diodes are more accessible but challenging in many modern panels where access is restricted. To pinpoint a faulty panel, particularly in larger arrays where it could be one among many, direct and indirect testing methods are useful. If heavy shading is visible on specific panels, these are likely starting points for troubleshooting. However, if no signs of faults are visible, systematic testing may be necessary to identify the problematic panel or panels within the string. Learn more in our detailed analysis of solar panel shading issues and the many problems associated with failing Bypass diodes.

How to know if your solar system has a problem?

If you believe your solar panels have a fault or the performance has noticeably decreased, there are several ways you can diagnose a problem. The first step is to visually check the solar panels for any signs of failure or dirt build-up, which can often result in mould growth and lead to poor performance. Often, a good wash with a soft broom and water is all that they need. Also, check no nearby trees have grown and are now shading the panels.

For systems with apps or web-based monitoring installed, it is often easy to compare the daily solar generation, measured in kWh, with previous days, weeks and months to see if there is a significant decline or change in performance. Note that the local climate, seasons and weather will also have an impact. Also, check the inverter for any obvious faults (red lights) and ensure that the isolator (circuit breaker) in the switchboard has not tripped off. For more information, see our detailed guide to solar system problems and fault finding.