Solar Panel Degradation: Rates, Warranties, and Failures

Solar panels lose a small fraction of their power output every year. This is called degradation, and it is a normal, expected process. Understanding the difference between normal degradation and a true failure helps in evaluating system performance and warranty claims.

What Is Degradation?

Degradation is the gradual decline in a solar panel’s power output over time. It is caused by a combination of factors: exposure to ultraviolet (UV) radiation, thermal cycling (heating and cooling), and moisture ingress. These stressors cause microscopic changes in the cell materials and the encapsulant that protects them. The result is a slow, steady reduction in the panel’s ability to convert sunlight into electricity.

The rate of degradation is typically expressed as a percentage of power loss per year. For modern crystalline silicon panels, the average annual degradation rate is around 0.5% to 0.8%. That means after 25 years, a panel might still produce 80% to 88% of its original rated power. This is not a defect; it is the natural behavior of the technology.

Performance Warranties: What They Promise

Most solar panel manufacturers offer a performance warranty, which guarantees that the panel will produce at least a certain percentage of its rated power after a specified number of years. For example, a common warranty might guarantee 90% of rated power after 10 years and 80% after 25 years. This is not a promise that the panel will not degrade; it is a promise that degradation will stay within a certain limit.

If a panel’s output falls below the guaranteed level, the manufacturer is obligated to replace or repair it. However, the warranty does not cover normal degradation that stays within the promised curve. It is crucial to read the warranty terms carefully, as they vary by manufacturer and may include conditions such as proper installation and maintenance.

Normal Degradation vs. Failure

A panel that has degraded is still functioning, just at a lower capacity. A panel that has failed is not functioning as designed, often due to a specific defect or event. Common causes of failure include:

• Microcracks: Tiny cracks in the silicon cells, often caused by thermal stress or mechanical impact, which can lead to power loss.
• Delamination: Separation of the layers of the panel, allowing moisture to enter and cause corrosion.
• Hot spots: Overheating of a small area, which can cause permanent damage.
• Diode failure: Bypass diodes, which protect the panel from hot spots, can fail, leading to reduced output.

These failures are usually not gradual; they cause a sudden drop in performance or a complete loss of output from a portion of the panel. They are often visible as discoloration, burn marks, or cracks on the panel surface.

How to Tell the Difference

If a system’s production is lower than expected, the first step is to check whether the degradation is within the normal range. This can be done by comparing the system’s current output to its original output, adjusted for changes in weather and irradiance. If the output is within the expected degradation curve, the system is performing as designed.

If the output is significantly below the expected curve, there may be a failure. A visual inspection can reveal obvious defects, but some failures are not visible. In that case, an I-V curve test can identify the issue. This test measures the current-voltage characteristics of the panel and can pinpoint problems such as microcracks or diode failures.

Common Misdiagnosis: Confusing Degradation with Failure

A common mistake is to assume that any drop in production indicates a failure. This is a natural assumption because the drop is noticeable and the system is not performing as it did when new. However, a small annual decline is normal and should be expected. For example, a system that produces 10,000 kWh in its first year might produce 9,500 kWh in its fifth year, assuming the same weather and conditions. That 5% drop is within the normal degradation range and does not indicate a problem.

Another misdiagnosis is attributing a production drop to degradation when it is actually caused by other factors, such as dirt, shading, or inverter issues. These can cause temporary or permanent losses that are not related to the panel’s age. Cleaning the panels or fixing the inverter can restore production, whereas degradation cannot be reversed.

The reason this misdiagnosis is common is that the symptoms are similar: the system produces less electricity than before. Without a clear understanding of degradation rates, it is easy to jump to the conclusion that the panels are failing. The key is to compare the actual output to the expected output based on the degradation curve.

Comparison Table: Degradation vs. Failure

What Affects Degradation Rate

The degradation rate is not the same for all panels. It depends on the manufacturing quality, the materials used, and the installation environment. Panels with better encapsulation and backsheet materials tend to degrade more slowly. Also, panels in hotter climates may degrade faster due to higher temperatures, which accelerate chemical reactions. Some panels have a lower temperature coefficient, meaning their performance is less affected by heat, but this does not directly change the degradation rate.

How Long Do Solar Panels Last?

Solar panels are designed to last 25 to 30 years or more. After that, they may still produce electricity, but at a reduced level. The lifespan is often defined by the performance warranty, which typically covers 25 years. However, many panels continue to function beyond that, albeit with lower output. The actual lifespan can be longer if the panel is well-maintained and not subjected to severe conditions.

What to Do If You Suspect Failure

If a system’s production drops sharply or a panel shows visible damage, it is advisable to contact a qualified solar installer or technician. They can perform an inspection and testing to determine the cause. Do not attempt to open or probe the DC wiring, as this is dangerous. Only licensed professionals should work on the electrical components.

Local Rules and Regulations

Net metering, interconnection, export compensation, and incentives are determined by state and utility policies, which change over time. These policies can affect the financial value of the system’s output and the payback period. Check with the local utility and state regulatory body for current rules. Do not assume that the rules in one jurisdiction apply to another.

What People Get Wrong About Degradation

One of the most common misconceptions is that a panel’s output will drop to zero after 25 years. In reality, it will still produce a significant amount of electricity, often around 80% of its original capacity. Another misconception is that degradation is a sign of poor quality. In fact, all panels degrade; the rate is what matters. A panel that degrades at 0.5% per year is performing well.

Another mistake is to compare the system’s output to its nameplate rating without accounting for real-world conditions. The nameplate rating is measured under standard test conditions (STC), which are not typical of everyday operation. Actual output is usually lower due to higher temperatures, lower irradiance, and other factors. So a system that produces 90% of its nameplate rating in the first year is not necessarily degraded; it is operating normally.

Conclusion

Degradation is a normal part of a solar panel’s life. Understanding the expected rate and what warranties cover helps in evaluating system performance. A sudden drop in output or visible damage indicates a failure, not degradation. By comparing actual output to the expected curve, one can determine whether a system is performing as designed. If in doubt, consult a professional.