Solar Panel Degradation and Warranties: What the Numbers Actually 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.