Quick Facts
What degradation means
Solar panel degradation is the slow permanent decline in a module’s electrical output over years of field operation. A new module at year zero produces its full nameplate Wp. The same module 10 years later produces a few percent less. After 25 years, output has dropped by 10% to 20% depending on technology and operating conditions.
Degradation is distinct from soiling, which is reversible through cleaning. It is also distinct from derating, which is the operational gap between nameplate and current output at any given moment. Degradation is the underlying long-term decline of the nameplate itself.
Manufacturers publish a linear performance warranty that specifies minimum output at year 25 (typically 80% to 90% of original nameplate). The warranty is a contractual commitment that compensates the owner if degradation exceeds the rated rate.
Mechanisms of degradation
Several physical and chemical processes contribute to long-term degradation.
UV exposure: Sunlight breaks down polymer materials (encapsulant, backsheet) over years. EVA encapsulant browns slightly under UV, reducing light transmission to the cells. POE encapsulant is more UV-stable than EVA.
Thermal cycling: Daily expansion and contraction stress solder joints and cell interconnects. Over years, microcracks form in cells and ribbons.
Moisture intrusion: Water vapour entering the module through edge seals corrodes contacts, hydrolyses encapsulant, and breaks down the backsheet. Glass-glass modules have better moisture resistance than polymer-backsheet.
Microcracking: Cells can develop hairline cracks during handling, transport, or thermal cycling. Cracks isolate parts of the cell, reducing collection.
Electromigration: Metal atoms in the cell contacts slowly migrate under operating current, increasing contact resistance and reducing fill factor.
LID (Light Induced Degradation): In p-type silicon, boron-oxygen complexes form under sunlight, reducing minority carrier lifetime. Loss is 1% to 3% in the first hours of field exposure for Mono PERC. N-type cells (TOPCon, HJT) do not have this LID.
LeTID (Light and elevated Temperature Induced Degradation): Slower form of degradation in some p-type PERC cells under hot conditions. Can add 0.5% to 1.5% per year in the first 5 years. Modern manufacturing largely mitigates this.
PID (Potential Induced Degradation): Driven by leakage currents in humid environments under voltage stress. Can cause significant output loss in affected modules. IEC 62804 tests for PID resistance.
Typical degradation rates
| Technology | First-year LID | Annual Degradation | Year 25 Output |
|---|---|---|---|
| Standard Mono PERC | 1% to 2% | 0.5% to 0.55% | 82% to 84% |
| Premium Mono PERC | 1% | 0.45% to 0.5% | 85% to 87% |
| TOPCon | around 1% | 0.4% | 87% to 89% |
| HJT | under 1% | 0.25% to 0.35% | 90% to 92% |
| Older polycrystalline | 2% to 3% | 0.7% to 0.8% | 75% to 80% |
These figures assume good operating conditions. Hot, humid, or coastal environments can accelerate degradation 1.5x to 2x for poorly designed modules.
Manufacturer warranties
Two warranties are typically offered.
Product warranty: Covers manufacturing defects, material failures, and workmanship. Typically 10 to 15 years, with premium products at 25 years.
Linear performance warranty: Guarantees minimum output at year 25 (or 30 for premium). The decline is assumed to be linear from year 1 to year 25.
A typical linear warranty for premium Mono PERC: 98% at year 1, declining linearly to 84.8% at year 25 (0.55% annual after year 1).
A typical linear warranty for premium TOPCon: 98% at year 1, declining linearly to 89.4% at year 30 (0.4% annual after year 1).
A typical linear warranty for premium HJT: 99% at year 1, declining linearly to 91.6% at year 30 (0.3% annual after year 1).
The warranty is enforceable against the manufacturer through field measurement. If actual output drops below the warranted level, the manufacturer provides replacement modules or compensation.
Real-world degradation patterns
Field studies in India and globally show that average degradation rates approximately match warranty levels for major manufacturers. However, significant variation exists:
Tier-1 manufacturers’ panels typically degrade at or below their warranted rates.
Non-Tier-1 panels often degrade faster, with field-measured rates of 0.8% to 1.2% per year not uncommon.
Hot tropical climates (Rajasthan, Gujarat industrial zones) see accelerated degradation versus temperate climates.
Coastal humid environments (Chennai, Mumbai, Vizag) see higher PID risk in cells without anti-PID protection.
Panels with cracks (from handling damage or hail) degrade much faster in the affected sections.
PVEL (PV Evolution Labs) scorecards and BloombergNEF Tier 1 lists help identify manufacturers with reliable long-term degradation performance.
How to slow degradation
Choose better technology. TOPCon and HJT degrade more slowly than Mono PERC. The lifetime energy advantage compounds across years.
Specify PID-resistant modules. IEC 62804 certification is the basic threshold. Coastal and humid sites should specifically require PID-resistant designs.
Use glass-glass modules where possible. Better moisture resistance than polymer-backsheet.
Ensure proper grounding and inverter configuration. Some PID stress comes from improper system grounding.
Minimise mechanical stress during installation. Cell cracks formed during installation cause faster degradation later.
Monitor regularly. Annual EL imaging or IV curve tracing catches early signs of accelerated degradation, allowing remedial action.
Use POE encapsulant instead of EVA for installations in humid environments.
Mount modules with adequate air gap for cooling. Lower operating temperatures slow degradation.
Common mistakes with degradation
Treating warranty figures as guaranteed performance. Warranties cover claims; real performance can be above or below the warranty curve.
Assuming all manufacturers degrade at the same rate. Variation between manufacturers can be 2x to 3x.
Skipping PID testing for coastal or humid Indian sites.
Ignoring the impact of installation damage. Microcracks from handling are a major degradation accelerator.
Using EVA encapsulant for glass-glass modules. EVA can produce acetic acid in long-term exposure, accelerating cell corrosion. POE is the better choice.
Forgetting that degradation compounds. A 1% annual rate after 25 years means 22% loss, not 25%.
Best practices
For long-term projects, choose modules with low degradation rates (TOPCon, HJT) and strong warranty terms.
Verify the manufacturer’s degradation history through PVEL scorecards and field studies.
Specify IEC 62804 PID certification in tenders for any project in humid or coastal regions.
Document module nameplate, serial numbers, and flash test reports at commissioning. These are the baseline for future warranty claims.
Conduct EL imaging and IV curve tracing every 3 to 5 years to track degradation trajectory.
For lender’s models, use conservative degradation rates (manufacturer rated rate plus 0.1% buffer) to account for real-world variation.
Standards and references
Degradation testing is covered in IEC 61215 (long-term thermal cycling, damp heat tests), IEC 61853 (energy yield characterisation), and IEC 62804 (PID specifically). Field measurement and warranty enforcement follow ASTM E2789 (degradation measurement) and IEC TS 63209 (long-term test method).
Related glossary terms
- Mono PERC
- TOPCon Solar Panel
- HJT Solar Panel
- PID and Anti-PID
- Performance Ratio
- Solar Derating
- IEC 61215 Standard
- N-type vs P-type
Key takeaways
Solar panel degradation is the slow permanent decline in module output over years. Annual degradation is 0.4% to 0.7% for modern modules, with TOPCon and HJT degrading more slowly than Mono PERC. A typical 25-year warranty guarantees at least 80% of nameplate output at year 25. Hot, humid, and coastal Indian conditions stress modules more than temperate climates. Choosing premium technology, PID-resistant designs, and careful installation all slow degradation and protect long-term plant economics.