Solar Panel Degradation & Warranties: What the Numbers Mean

Solar panels degrade over time — the semiconductor materials in the cells gradually lose efficiency under UV exposure, thermal cycling, and humidity. A small amount of degradation is normal and expected; what matters is the rate, because it compounds over 25 years into a meaningful difference in total generation. The spec sheet figures and warranty terms that describe this are worth understanding before you sign off on a quote.

Annual degradation: what the percentage means

Panels typically lose a larger fraction of their output in the first year (often cited as 1–2%) due to light-induced degradation (LID), and then settle into a slower annual rate for the remainder of their life. The warranty figure you will see quoted is the steady-state annual degradation rate after the first year.

Standard PERC panels typically warrant less than 0.55% per year. Better TOPCon and N-type panels typically warrant less than 0.4% per year. The difference sounds small — 0.15 percentage points — but it compounds.

Take a 400W panel. After 25 years at 0.55%/yr degradation: 400W × (1 − 0.55% × 25) = roughly 345W at end of life. At 0.4%/yr: 400W × (1 − 0.4% × 25) = roughly 360W. That is a 15W difference on a single panel, or around 150W on a 10-panel system — roughly 4% more generation in year 25. Across the full 25-year period, a lower degradation rate adds up to perhaps 2–3% more total lifetime energy, which is not the primary reason to choose a panel but is a genuine long-term benefit.

Product warranty vs performance warranty: different things

This is the distinction that catches people out. A product warranty (sometimes called a materials or workmanship warranty) covers manufacturing defects — panels delaminating, junction box failures, frame corrosion. Most manufacturers offer 10–15 years product warranty; better ones offer 25 years. This is insurance against the panel physically failing.

A performance warranty is a separate promise about minimum output over time. A typical performance warranty says the panel will produce at least 80% of its rated output after 25 years. Better warranties promise 85–87% at 25 years. This figure is directly related to the annual degradation rate: a panel warranted at 85% after 25 years is implicitly warranting degradation of no more than 0.6% per year on average (with first-year loss factored in).

Both warranties matter, but they cover different risks. A panel can have a good performance warranty and a short product warranty — meaning the manufacturer is confident in output degradation but less willing to warrant against physical defects. Check both figures separately.

How warranty reflects cell type

The better degradation warranties are not accidental — they correlate closely with cell technology. N-type cells (TOPCon, HJT) are inherently more resistant to LID and LeTID, which is why manufacturers can warrant lower degradation rates. When you see a 0.4%/yr or better degradation warranty, you are almost certainly looking at an N-type panel. This is another reason why the cell type discussion matters practically — see our guide to solar panel cell types for the full picture.

What to ask your installer

Before accepting a quote, ask for both the product warranty term and the end-of-life performance guarantee (as a % of rated output at year 25). If the installer cannot tell you both figures from the spec sheet, that is a gap worth closing before you sign. Browse solar panels on SmartSolarHomes to compare degradation warranties and performance guarantees side by side.

Cleaning method also matters for warranty compliance. Most manufacturers permit soft-bristle brushes and deionised water but explicitly prohibit pressure washers and abrasive pads — using them can void both the product and performance warranty. Our guide to solar panel cleaning in the UK covers the safe DIY method and what to avoid.

Physical damage from wildlife is another underappreciated warranty risk. Pigeons nesting under panels can peck DC cables and connectors, potentially voiding workmanship warranties through damage caused after installation. Fitting a pigeon proofing mesh skirt is a low-cost preventive step that protects both the cabling and your warranty claims.

UK solar panel degradation rates by brand

Not all panels degrade at the same rate. Manufacturers publish annual degradation rates in their datasheets, and the figures vary meaningfully across the market. At UK average irradiation of 850–950 kWh/kWp per year, the difference between a 0.25% and 0.55% annual degradation rate compounds into a significant output gap over 25 years.

The difference matters: a 400 W REC panel at 0.25% annual degradation produces roughly 8,840 kWh over 25 years (at UK average irradiation). The same-rated panel at 0.55% annual degradation produces approximately 8,230 kWh — around 610 kWh less, worth about £150 at current energy prices.

25-year generation impact: a worked example

Take a 4 kWp system installed in the UK Midlands (approximately 900 kWh/kWp/yr in year one). Here is how annual generation compares between a low-degradation and standard-degradation panel:

• Year 1: 3,600 kWh (both panels, same starting output)
• Year 10 (0.25%/yr): ~3,510 kWh | Year 10 (0.55%/yr): ~3,420 kWh
• Year 25 (0.25%/yr): ~3,380 kWh | Year 25 (0.55%/yr): ~3,110 kWh
• 25-year total difference: approximately 2,450 kWh — worth roughly £600 at 24p/kWh

This is worth factoring into any comparison between a cheaper panel with higher degradation and a premium panel with a lower degradation warranty, rather than comparing only upfront cost.

Common warranty exclusions

Solar panel warranties — both the product warranty and the performance warranty — contain exclusions that are rarely discussed at the point of sale. Understanding these before installation can prevent expensive disputes later.

• Bird and pest damage: Droppings causing micro-cracks, or nesting damage to wiring, is almost universally excluded from product warranties. Pigeon proofing mesh significantly reduces this risk.
• Non-approved mounting: Installing panels on a mounting system not specified or approved by the manufacturer can void the product warranty. Check your installer is using a compatible racking system.
• DIY cleaning damage: Using abrasive cleaners, pressure washers at the wrong distance, or sharp implements to remove moss typically voids the warranty for any resulting damage. Always use soft cloths and appropriate cleaning solutions.
• Modifications: Any modification to the panel itself — drilling, trimming, or bypassing the junction box — will void cover.
• Acts of nature (varies): Hail, lightning strikes, and storm damage are typically covered by your home insurance rather than the panel warranty. Confirm with your insurer before installation that your solar system is included in your buildings cover.

How to make a warranty claim in the UK

If you believe a panel is performing below its warranted output, the process is more involved than a standard product return. Here is what is typically required:

1. Gather monitoring data: Pull at least 12 months of generation data from your inverter app or monitoring portal. You need to demonstrate consistent underperformance, not just a bad day.
2. Compare against PVGIS estimates: Use the EU Joint Research Centre PVGIS tool to model expected output for your specific postcode, tilt, and orientation. This provides the baseline your claim is measured against.
3. Contact your installer first: Most warranties require you to contact the original MCS-certified installer, not the manufacturer directly. Your installer should carry out an inspection and provide a report.
4. Escalate to the manufacturer: If the installer confirms underperformance beyond warranty limits, the claim is submitted to the manufacturer with the inspection report. Manufacturers typically send an engineer to verify before agreeing to replace panels.
5. RECC or HIES as backstop: If your installer has ceased trading, the Renewable Energy Consumer Code (RECC) or HIES scheme provides warranty protection. Your MCS certificate will confirm which consumer code covered your installation.