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Equipment · updated October 6, 2026

Panel degradation and warranties

What 0.5% a year means over 25 years, the difference between a product and a performance warranty, and what to look for in the fine print.

Solar panels slowly lose output. The calculator assumes 0.5% per year, a common planning figure that most panel warranties are written to beat. Here is what that compounds to and what the paperwork actually promises.

The arithmetic

  • Year 10: about 95.6% of year-one output.
  • Year 25: about 88.7%.
  • Averaged across all 25 years: about 94.2% of year-one output. So a system that makes 10,000 kWh in year one makes roughly 2,356 kWh over 25 years, not 250,000.

The year-by-year table under the calculator shows this decline applied to your own system alongside rising utility prices. The two effects pull in opposite directions; in most Virginia scenarios rising prices win, which is why annual savings in dollars tend to grow even as kWh shrink.

Two warranties, not one

Product (materials and defects) warranty

From the panel manufacturer. Covers defects: delamination, failed connectors, cracked glass from manufacturing faults. Historically 10 to 12 years; many current panels carry 25. Longer is better, but only if the manufacturer will still exist to honor it. The installer's separate workmanship warranty covers the installation itself: mounting, wiring, roof penetrations.

Performance (power output) warranty

Promises the panel will still produce at least X% of its rated output at year 25 (sometimes 30), with a schedule in between. Figures in the high 80s to low 90s percent are typical for current tier-one panels, which implies a degradation rate at or below the 0.5% used here. Virtue Solar's degradation explainer walks through real spec sheets.

What the warranty does not cover

  • Shade, soiling, snow. Site conditions are yours.
  • The inverter. Separate warranty, usually 10 to 12 years for string inverters and 25 for microinverters. Budget for one string-inverter replacement around year 12 to 15; the calculator does not include it, and the methodology page says so.
  • Labor to remove and replace, unless the installer's workmanship warranty covers it. Ask.
  • Roof penetrations. Covered by the installer's workmanship warranty, typically 10 years or more. Ask what happens if the roof needs replacing mid-life.

How to read a spec sheet in 30 seconds

  1. Find "linear power warranty" or "performance guarantee." Note first-year degradation (often around 1 to 2%) and the annual rate thereafter.
  2. Find the year-25 guaranteed output. Higher is better: a panel guaranteed 92% at year 25 instead of 85% is promising about 8% more output in that final year, and roughly 5% more averaged over years 16 to 25.
  3. Check who backs it: the manufacturer, a third-party insurer, or the installer. Two of the three is comfortable.