Solar panel degradation: what to expect from your Perth system over 25 years

Every solar panel loses efficiency over time. This is well-understood, quantified in performance warranties, and a normal part of operating a solar system over 25 years. What matters is whether your system is degrading at the expected rate or faster.

Here's what the numbers actually mean.

How solar panels degrade

Panel output declines due to several mechanisms:

Light-Induced Degradation (LID): In the first few days to weeks of exposure to sunlight, most traditional PERC (Passivated Emitter Rear Cell) panels lose 1–3% of their rated output. This is caused by boron-oxygen defects activated by light. It's permanent but expected — panel manufacturers test for it and their rated output at STC (Standard Test Conditions) already accounts for some of this, but initial real-world output is often 1–3% below nameplate after the first few weeks.

Long-term degradation: After the initial LID, panels degrade at a slower, relatively steady rate. Typical annual degradation rates:

• Premium panels (REC, Q CELLS, most N-type TOPCon): 0.3–0.4%/year
• Standard PERC panels: 0.4–0.55%/year
• Older or lower-quality panels: 0.5–0.8%/year

N-type TOPCon panels (Jinko Neo, Trina Vertex N, Canadian Solar HiKu series) are largely immune to LID because they don't use boron-doped silicon. Their initial output is more stable, and their long-term degradation is at the lower end (~0.3–0.4%/year).

Light and Elevated Temperature Induced Degradation (LETID): A secondary mechanism causing additional degradation in PERC panels under prolonged thermal stress. More relevant in hot climates — which Perth definitely is. Manufacturers have largely addressed LETID in panels produced after 2019 via annealing processes during manufacturing.

Reading your performance warranty

Performance warranties guarantee minimum output at a specific year:

Standard warranty structure:

• End of year 1: 97% of rated output (allows for LID)
• End of year 25: 80% of rated output

Premium warranty structure:

• End of year 1: 98% of rated output
• End of year 25: 84–92% of rated output (varies by manufacturer)

What this means:

If you have a 6.6kW system with a 25-year / 80% warranty, the manufacturer guarantees at least 5.28kW of output capacity at year 25.

The implied annual degradation for a 97% → 80% warranty over 24 years is:

(97% - 80%) / 24 years = 0.71% per year

But well-made panels degrade at 0.4–0.5%/year, so at year 25 they'd be at approximately 87–90% — well above the 80% floor. The 80% warranty is a floor, not a prediction.

Better panels often offer warranties that more accurately reflect expected performance:

• REC Alpha: 92% at 25 years (implied 0.21%/year — extremely conservative)
• Q CELLS Q.PEAK DUO: 86% at 25 years
• Canadian Solar (TOPCon series): 87.4% at 25 or 30 years

What your system output looks like over time

6.6kW system, 0.5%/year degradation, Perth PSH 5.0:

| Year | Panel capacity | Annual generation | |---|---|---| | Year 1 | 6.6kW (100%) | ~12,000 kWh | | Year 5 | ~6.44kW (97.5%) | ~11,700 kWh | | Year 10 | ~6.27kW (95%) | ~11,400 kWh | | Year 15 | ~6.11kW (92.5%) | ~11,100 kWh | | Year 20 | ~5.94kW (90%) | ~10,800 kWh | | Year 25 | ~5.77kW (87.5%) | ~10,500 kWh |

A well-maintained system at year 25 generates approximately 12.5% less than in year 1, assuming no component failures. This gradual reduction is spread over decades — year-on-year you'd barely notice it.

Inverter replacement is more likely to disrupt generation over a 25-year period than panel degradation. String inverters typically need replacement at 10–15 years ($600–$1,500). Budget for this when considering whole-system lifetime cost.

Perth-specific factors

Temperature stress: Perth summer roof temperatures regularly reach 60–75°C. Higher operating temperatures accelerate some degradation mechanisms. This is one reason N-type panels perform better in Perth than PERC panels — better thermal stability.

UV exposure: Perth has high UV intensity, particularly in summer. UV can accelerate encapsulant yellowing in older panels, which affects light transmission. Premium panels use UV-stable encapsulants; cheaper panels are more susceptible.

Dust and soiling: Regular cleaning (discussed in our solar maintenance guide) prevents build-up that compounds apparent degradation. A 10% soiling loss looks similar to years of degradation in your monitoring data — clean panels before drawing conclusions about degradation.

When a generation drop is normal vs a problem

Normal:

• Gradual year-on-year decline of 0.3–0.7% per year (check monitoring data on the same-month-previous-year basis)
• Seasonal variation (winter vs summer can be 50% different in daily generation)
• Day-to-day variation from weather and cloud cover

Worth investigating:

• A sudden drop of 10%+ on clear days that persists for several weeks (rule out soiling first by cleaning panels)
• One panel consistently producing 30%+ less than others in monitoring data (microinverter/optimiser systems show per-panel data; string inverters show total)
• Generation consistently 20%+ below the installer's estimate on clear days after accounting for seasonal adjustment
• Inverter warning codes that don't clear on restart

Warranty claim triggers:

• Measured output (not monitoring estimate — use a proper irradiance-adjusted measurement) below the warranted percentage for the panel age
• Visible delamination, cell cracking visible through the glass, browning or yellowing of cells

Making a performance warranty claim requires professional irradiance-corrected testing — the monitoring app estimates are not precise enough for warranty purposes. An accredited installer can conduct an IV curve test to measure actual panel performance against rated specifications.

Degradation rates are based on published research and manufacturer data for panels sold from 2019 onwards. Older panels (pre-2018 PERC, pre-2015 poly-crystalline) may have higher actual degradation rates than shown. Your monitoring app's year-on-year comparison is the most accessible proxy for tracking degradation.