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

Every solar panel loses efficiency over time. For a 6.6 kW system in Perth, a well-made panel degrading at 0.5%/year will still deliver around 87–88% of its original output at year 25, generating roughly 8,900–9,000 kWh/year instead of the ~10,200 kWh/year it produced on day one (Perth PSH 5.0, 0.85 system derate).

Year-on-year you'd barely notice it. The real question is whether your system is degrading at the expected rate, or faster.

How solar panels degrade

Three mechanisms drive most of the loss.

Light-induced degradation (LID): In the first days to weeks of sunlight exposure, most traditional PERC (Passivated Emitter Rear Cell) panels lose 1–3% of their rated output. Boron-oxygen defects activated by light cause this. It's permanent but expected. Manufacturers test for it, and initial real-world output is often 1–3% below nameplate after the first few weeks.

Long-term degradation: After that initial drop, panels degrade at a slower, relatively steady rate. Typical annual figures:

• Premium panels (REC, LONGi, 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

(0.5%/year is used here as the standard default.)

Jinko Neo, Trina Vertex N, and Canadian Solar HiKu N-type TOPCon panels largely avoid LID because they don't use boron-doped silicon. Initial output is more stable and long-term degradation sits 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 heat stress, relevant in Perth given roof temperatures that regularly reach 60–75°C in summer. How well LETID is mitigated varies by manufacturer, cell design, and production batch, so check a specific product's technical documentation rather than assuming it has been eliminated.

See also: PERC vs TOPCon vs HJT solar panels in Perth for a side-by-side on technology differences.

Reading your performance warranty

Performance warranties guarantee a minimum output at a specific year. Two tiers are common.

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)

For a 6.6 kW system with a 25-year / 80% warranty, the manufacturer guarantees at least 5.28 kW 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

Well-made panels typically degrade at 0.4–0.5%/year, so at year 25 they'd sit around 87–90%. That's well above the 80% floor, which is a safety net, not a prediction.

Examples from manufacturer datasheets:

• REC Alpha (HJT): 92% of nameplate at year 25
• LONGi: the Distributed Generation performance warranty runs to 30 years on eligible module families; the specific end-of-life output depends on the exact model and schedule, so check the datasheet for your module
• Canadian Solar (Australia): the KU-series Australian warranty guarantees 84.8% of nameplate at year 25; Australian double-glass modules guarantee 83.0% or 81.5% at year 30 depending on family. The global TopHiKu / TopBiHiKu "87.4% at 30 years" figure explicitly excludes Australia and New Zealand, so it doesn't apply to AU buyers

What your system output looks like over time

6.6 kW system, 0.5%/year degradation, Perth PSH 5.0 (0.85 system derate)

Figures calculated as sizeKw × PSH(5.0) × 0.85 × 365 (Perth PSH 5.0, 0.85 derate).

That's a 12.5% reduction over 25 years on a well-maintained system. These are illustrative estimates. Your actual output depends on shading, roof pitch, orientation, and system condition. The BillWise solar output by month guide shows how seasonal variation adds to this picture.

Panels aren't usually what disrupts generation over 25 years. Inverters are. String inverters typically need replacement at 10–15 years. Budget for that when you're thinking about whole-system lifetime cost; get a current quote from your installer or see our inverter lifespan guide for Perth for replacement-cost context.

Perth-specific factors

Temperature stress: Roof temperatures regularly hit 60–75°C in summer. Higher operating temperatures accelerate some degradation mechanisms. N-type panels handle this better than PERC. Thermal stability is a real part of why they degrade more slowly here.

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

Dust and soiling: A 10% soiling loss looks similar to years of degradation in monitoring data. Clean your panels before drawing conclusions about performance. See our solar maintenance guide for the basics.

When a generation drop is normal vs a problem

Normal:

• Gradual year-on-year decline of 0.3–0.7% (check monitoring data on a same-month-previous-year basis)
• Seasonal variation (winter vs summer daily generation in Perth can differ by around 50%)
• 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)
• One panel consistently producing 30%+ less than others (microinverter and optimiser systems show per-panel data; string inverters show total only)
• Generation consistently 20%+ below the installer's estimate on clear days, after seasonal adjustment
• Inverter warning codes that don't clear on restart

See how to read your solar monitoring data for a step-by-step on spotting real performance issues.

Warranty claim triggers:

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

Monitoring app figures aren't precise enough for a warranty claim. You need professional irradiance-corrected testing: an accredited installer can run an IV curve test to measure actual panel performance against rated specs. The solar warranty claims guide covers the process in detail.

---

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 show higher actual degradation than shown. Generation figures use Perth PSH 5.0 and 0.85 system efficiency derate. Your monitoring app's year-on-year comparison is the most accessible proxy for tracking real-world degradation.