What Is Solar Module Degradation? Causes, Types, Rate and How It Happens
Solar module degradation is the gradual reduction in the electrical performance of a solar module over time. As a solar panel operates outdoors for many years, sunlight, UV radiation, heat, humidity, mechanical stress and other environmental conditions can gradually affect its performance.
A new solar module has a specified power rating under defined test conditions. For example, a module may have a rated power of 550 W. After several years of outdoor operation, its maximum power may be lower than when it was new.
Degradation is a normal part of the operating life of a photovoltaic (PV) module. However, the amount and speed of degradation are not the same for every module. They depend on the cell technology, module construction, materials, environment, operating conditions and the specific degradation mechanism involved.
Key Takeaways
- Solar module degradation is the gradual loss of electrical performance over time.
- A commonly cited historical reference value is around 0.5% degradation per year.
- Actual degradation rates vary between technologies, modules and environments.
- LID, LeTID and PID are important degradation mechanisms.
- UV radiation, heat, humidity and mechanical stress can contribute to degradation.
- Cell cracking, delamination, corrosion and encapsulant degradation can also affect long-term performance.
- Long-term degradation should be evaluated using appropriate electrical and field-performance measurements.
- Why Does Solar Module Degradation Matter?
- How Does Solar Module Degradation Happen?
- Main Types of Solar Module Degradation
- Light-Induced Degradation (LID)
- Light and Elevated Temperature-Induced Degradation (LeTID)
- Potential-Induced Degradation (PID)
- UV-Induced Degradation
- Thermal Stress and Temperature Cycling
- Cell Cracking
- Encapsulant Degradation
- Delamination
- Corrosion
- Typical Solar Module Degradation Rate
- Example of 0.5% Annual Degradation
- How Is Solar Module Degradation Measured?
- Can Solar Module Degradation Be Prevented?
- Degradation vs Temporary Power Loss
- Frequently Asked Questions
- Conclusion
Why Does Solar Module Degradation Matter?
A small percentage of power loss every year may appear insignificant. However, solar PV modules are designed to operate for many years, so even a small annual degradation rate can have a noticeable effect over the lifetime of a project.
Degradation can affect annual electricity generation, lifetime energy production and the long-term performance of a solar installation.
How Does Solar Module Degradation Happen?
A solar module contains several materials and components that work together. These include solar cells, glass, encapsulant, backsheet or rear glass, interconnections, junction box, bypass diodes, frame, sealants and adhesives.
During outdoor operation, these materials are continuously exposed to environmental stress.
- Sunlight: Long-term exposure to solar radiation can affect materials.
- UV radiation: Can affect polymeric materials used in the module.
- Heat: High operating temperatures can accelerate some degradation mechanisms.
- Humidity: Moisture can contribute to corrosion and material degradation.
- Mechanical stress: Wind, installation and thermal movement can stress cells and interconnections.
- Electrical stress: Certain electrical conditions can contribute to mechanisms such as PID.
What Are the Main Types of Solar Module Degradation?
Solar module degradation is not a single mechanism. Different mechanisms can affect different parts of a PV module.
- Light-Induced Degradation (LID)
- Light and elevated Temperature-Induced Degradation (LeTID)
- Potential-Induced Degradation (PID)
- UV-induced degradation
- Thermal stress and temperature cycling
- Cell cracking
- Encapsulant degradation
- Delamination
- Corrosion
1. Light-Induced Degradation — LID
LID stands for Light-Induced Degradation. It describes a reduction in PV performance that occurs after a solar cell is exposed to light.
LID can occur relatively early in the operating life of certain silicon PV technologies. It is associated with material and defect interactions inside the solar cell.
When the cell is exposed to light, certain defect-related processes can change its electrical characteristics, resulting in a reduction in parameters such as maximum power.
2. Light and Elevated Temperature-Induced Degradation — LeTID
LeTID stands for Light and elevated Temperature-Induced Degradation. It is a degradation mechanism associated with certain silicon PV technologies.
As its name suggests, light exposure and elevated temperature are important conditions associated with this mechanism.
LeTID can be important when evaluating long-term PV performance because degradation behaviour can depend on temperature and exposure conditions.
3. Potential-Induced Degradation — PID
PID stands for Potential-Induced Degradation. PID is associated with electrical potential differences between parts of a PV module and the electrical system.
Under certain combinations of system voltage, temperature, humidity, module construction, cell technology and encapsulation materials, electrical performance can deteriorate.
PID can result in a noticeable reduction in module power. The exact mechanism can vary depending on the cell and module construction.
4. UV-Induced Degradation
Solar radiation contains ultraviolet radiation, commonly called UV radiation. Continuous UV exposure can affect polymeric materials used in solar modules.
These materials can include:
- Encapsulants
- Backsheets
- Adhesives
- Other polymer components
Long-term UV exposure can contribute to discoloration, yellowing, loss of optical properties, material deterioration and, in some cases, delamination.
If optical properties change, less sunlight may reach the solar cells, which can affect the module's electrical output.
5. Thermal Stress and Temperature Cycling
Solar modules heat up during the day and cool down when the temperature falls. This repeated temperature change is called thermal cycling.
Different module materials expand and contract at different rates. Repeated expansion and contraction can create mechanical stresses inside the module.
Over many cycles, this can contribute to:
- Solder fatigue
- Interconnection stress
- Cell cracking
- Encapsulant stress
- Delamination
6. Cell Cracking
Solar cells are thin and relatively fragile. Mechanical stress can cause cracks in the silicon cell.
Possible causes include:
- Transportation
- Installation
- Wind loading
- Thermal cycling
- Mechanical loading
- Module deformation
Not every crack produces the same amount of power loss. Depending on its location, a crack can electrically isolate part of a cell and reduce the effective active area.
7. Encapsulant Degradation
The encapsulant surrounds and protects the solar cells. It provides mechanical protection, electrical insulation, adhesion and optical transmission.
Over time, the encapsulant can undergo material changes such as yellowing, browning, loss of transparency, loss of adhesion and moisture-related deterioration.
If the encapsulant becomes less transparent, the amount of sunlight reaching the solar cells can decrease.
8. Delamination
Delamination means separation between layers of a solar module.
For example, separation can occur between:
- Glass and encapsulant
- Encapsulant and backsheet
- Encapsulant and other module layers
Delamination can create pathways for moisture to reach sensitive components. It can also affect optical and mechanical properties.
9. Corrosion
Moisture and environmental exposure can contribute to corrosion of metallic components inside a solar module.
Potentially affected components include:
- Cell metallization
- Ribbons
- Interconnections
- Electrical contacts
Corrosion can increase electrical resistance and contribute to power loss.
What Is the Typical Solar Module Degradation Rate?
There is no single degradation rate that applies to every solar panel.
NREL's review of nearly 2,000 published degradation rates reported a median degradation rate of approximately 0.5% per year.
However, this should be treated as a reference value rather than a guaranteed degradation rate for every modern solar module.
Actual degradation depends on factors such as:
- Cell technology
- Module design
- Materials
- Climate
- Installation conditions
- Operating conditions
- Age
- Specific degradation mechanism
Example: How Does 0.5% Annual Degradation Affect a 550 W Module?
Suppose a solar module has an initial rated power of 550 W. If we use a simplified constant annual degradation assumption of 0.5%, the approximate power can be calculated using:
| Operating Period | Illustrative Power |
|---|---|
| Initial | 550 W |
| 1 year | 547.25 W |
| 5 years | ≈ 536.4 W |
| 10 years | ≈ 523.1 W |
| 20 years | ≈ 497.6 W |
| 25 years | ≈ 485.3 W |
Important: This is a mathematical illustration using a constant 0.5% annual rate. Actual solar module degradation is not necessarily perfectly linear.
Does a Solar Panel Lose 0.5% Power Every Year?
Not necessarily.
This is an important point because real degradation does not always follow a perfectly straight line.
A module may experience a relatively larger initial loss followed by a lower degradation rate, or its degradation behaviour may change with operating conditions and age.
Mechanisms such as LID and LeTID can also influence early-life performance in susceptible technologies.
Solar Module Degradation vs Temporary Power Loss
Not every reduction in solar output means that the module has degraded.
| Condition | Typical Effect |
|---|---|
| Dust / soiling | Can temporarily reduce output |
| Cloud cover | Temporarily reduces generation |
| High operating temperature | Can temporarily reduce operating power |
| Shading | Reduces generation while shading exists |
| Permanent degradation | Long-term reduction in module performance |
Therefore, if a solar panel produces less power on a particular day, it does not automatically mean that the module has degraded.
How Is Solar Module Degradation Measured?
1. IV Curve Measurement
An IV curve shows the relationship between current and voltage. Important parameters include:
- Voc: Open-circuit voltage
- Isc: Short-circuit current
- Vmp: Voltage at maximum power
- Imp: Current at maximum power
- Pmax: Maximum power
Comparing these parameters over time can help identify changes in module performance.
2. Electroluminescence — EL Testing
Electroluminescence testing can help identify physical problems inside solar cells, including cracks, inactive regions and certain interconnection abnormalities.
3. Visual Inspection
Visual inspection can identify signs such as:
- Discoloration
- Delamination
- Cracked glass
- Backsheet damage
- Corrosion
- Burn marks
- Junction-box damage
Visual inspection alone, however, cannot determine the complete electrical degradation of a module.
4. Long-Term Field Monitoring
Long-term field monitoring can be used to evaluate performance changes over time. Proper analysis needs to account for irradiance, temperature, soiling, availability, seasonal effects and measurement uncertainty.
Can Solar Module Degradation Be Prevented?
Solar module degradation cannot be completely eliminated because PV modules naturally age during long-term outdoor operation.
However, the rate of degradation can be reduced through appropriate cell technology, material selection, module design, installation and operating conditions.
- Use suitable cell technology: Different technologies have different degradation characteristics.
- Use reliable materials: Encapsulants, backsheets, glass and interconnections affect durability.
- Use suitable module design: Proper construction helps protect cells and electrical components.
- Install modules correctly: Correct handling and mounting reduce unnecessary mechanical stress.
- Consider the environment: Temperature, humidity, UV exposure and coastal conditions should be considered when selecting modules.
Why Do Solar Modules Have Different Degradation Rates?
Solar technology has changed significantly over the years. Different modules can use different silicon wafers, cell structures, metallization systems, encapsulants, backsheets, interconnection designs and module architectures.
As a result, degradation performance can differ significantly between module technologies and individual products.
Simple Example of Solar Module Ageing
Imagine installing a new 550 W solar module today.
During its operating life, the module is exposed to sunlight, UV radiation, heat, cooling, humidity and mechanical stress.
Over many years, some degradation can occur. The module may continue generating electricity normally, but its maximum power may be lower than when it was new.
Solar Module Degradation vs Module Failure
Degradation and failure are not the same thing.
| Term | Meaning |
|---|---|
| Degradation | Gradual reduction in performance over time. |
| Failure | A condition where a module or component can no longer perform its intended function or meet the applicable requirement. |
Therefore, a solar module can degrade without completely failing.
Why Low Degradation Is Important
Consider two otherwise similar modules:
- Module A: 0.3% annual degradation
- Module B: 0.8% annual degradation
The difference in one year may look small. Over several decades, however, the cumulative effect can become significant.
A lower-degradation module can potentially deliver more energy over its lifetime. This is why degradation is an important consideration when evaluating long-term PV performance.
Key Factors That Influence Solar Module Degradation
| Factor | Possible Effect |
|---|---|
| UV radiation | Polymer and material degradation |
| High temperature | Can accelerate some degradation mechanisms |
| Thermal cycling | Mechanical stress and interconnection fatigue |
| Humidity | Corrosion and moisture-related degradation |
| Mechanical stress | Cell cracking |
| Electrical potential | PID risk under suitable conditions |
| Cell/material properties | LID and LeTID susceptibility |
| Encapsulant properties | Optical and adhesion-related degradation |
| Module construction | Influences long-term durability |
| Climate | Changes environmental stress exposure |
Frequently Asked Questions
What is solar module degradation?
Solar module degradation is the gradual reduction in a PV module's electrical performance as it ages and experiences environmental and operating stresses.
What is the average solar panel degradation rate?
Around 0.5% per year is a commonly cited historical reference value, but actual degradation varies between technologies, products and operating environments.
Do all solar panels degrade at the same rate?
No. Degradation depends on cell technology, materials, module design, environment and operating conditions.
What causes solar panel degradation?
Major causes include light exposure, UV radiation, temperature changes, humidity, mechanical stress, LID, LeTID, PID, cell cracking, encapsulant degradation, delamination and corrosion.
What is LID in solar panels?
LID, or Light-Induced Degradation, is a reduction in PV performance associated with exposure to light in susceptible solar-cell technologies.
What is LeTID?
LeTID means Light and elevated Temperature-Induced Degradation. It is a degradation mechanism associated with certain PV technologies under light and elevated-temperature conditions.
What is PID in solar modules?
PID, or Potential-Induced Degradation, is a performance-loss mechanism associated with electrical potential differences and specific environmental and module conditions.
Can solar panel degradation be stopped?
It cannot be completely stopped, but appropriate cell technology, materials, module design, installation and operating conditions can help reduce degradation.
Does heat permanently reduce solar panel power?
High temperature can cause an immediate operating-power reduction according to the module's temperature characteristics. This temporary temperature effect should not automatically be confused with permanent degradation.
How can solar module degradation be measured?
Long-term degradation can be evaluated using repeated electrical measurements, field-performance data and appropriate analysis. EL and visual inspections can help identify physical degradation mechanisms.
Do solar panels stop working after 25 years?
Not necessarily. A module can continue generating electricity beyond 25 years, although its output may be lower than when it was new.
Conclusion
Solar module degradation is the gradual loss of PV module performance over time. It occurs because solar modules spend years exposed to sunlight, UV radiation, temperature changes, humidity, mechanical stresses and electrical stresses.
Important degradation mechanisms include LID, LeTID, PID, UV degradation, thermal stress, cell cracking, encapsulant degradation, delamination and corrosion.
A degradation rate of approximately 0.5% per year is a commonly used historical reference, but it should not be treated as a universal value for every modern PV module. Actual degradation depends on the technology, module construction, materials and operating environment.
