The Vermont attic
There’s a solar technician named Linda climbing into a 22-year-old solar installation in Vermont. The homeowner has called her because they think the system is losing too much output. Linda runs an IV-curve trace on each panel. The results: median 0.4%/year degradation. The system is producing 91% of its original output at year 22. The homeowner had been told to expect 80% at year 25. They’re doing better than expected by about 11 percentage points, which over 22 years has been about 4,000 kWh of “free” extra electricity they didn’t know they had.
Solar panel degradation is a story most people tell wrong. The 0.5%/year figure quoted everywhere is a median across a noisy distribution. Some panels degrade much faster. Some much slower. And almost nobody can tell you why their specific panels are doing what they’re doing without actually measuring them.
The science: what actually causes degradation
Solar panels lose output for four main reasons, and each has its own physics:
Light-Induced Degradation (LID): A small efficiency drop (1–3%) in the first 100–1000 hours of sun exposure for crystalline silicon panels. Built into the rated output. PERC panels have a variant called LeTID (light and elevated Temperature Induced Degradation) that can cause 5–10% drop if not handled by the manufacturer’s stabilization process. Tier-1 manufacturers stabilize their panels; Tier-2 sometimes don’t.
Potential-Induced Degradation (PID): Voltage stress between the panel and frame can cause sodium ions from the glass to migrate, reducing cell efficiency. Modern panels include PID-resistant designs (anti-PID encapsulants, modified cell architectures). Affects mostly panels installed before 2015.
Thermal cycling and microcracks: Panels expand and contract with daily temperature swings. Over years, microcracks develop in the silicon. They start invisible, grow over time, and eventually create hot spots that accelerate further degradation. Modern half-cut and shingled cells distribute mechanical stress better.
Encapsulant browning and delamination: The EVA polymer encapsulating cells yellows with UV exposure and heat. Modern EVA formulations are dramatically better than 1990s versions. POE (polyolefin elastomer) encapsulants used in premium glass-glass panels are even better.
The numbers
Median degradation rates from NREL’s long-running field studies (2024 updates):
| Panel category | Median degradation rate | What this means at year 25 |
|---|---|---|
| Premium mono (Maxeon, REC Alpha, SunPower) | 0.25–0.40%/year | 91–94% output |
| Tier-1 mono (Jinko, Trina, Longi, Canadian Solar) | 0.40–0.55%/year | 87–90% output |
| Older poly (pre-2015) | 0.60–1.00%/year | 78–86% output |
| Tier-2 / unknown brand mono | 0.50–0.80%/year | 81–88% output |
| Thin-film (CdTe, modern) | 0.40–0.50%/year | 88–90% output |
| Amorphous silicon | 1.0–1.5%/year | 69–78% output |
These are medians across thousands of installations. Individual panels can fall well above or below. The spread is wide enough that buying premium isn’t a guarantee and buying budget isn’t a death sentence — it shifts the odds.
Why some panels degrade faster
Three biggest factors in real-world degradation:
- Climate: Hot climates degrade panels faster. Phoenix vs Berlin can mean 0.7%/year vs 0.4%/year on the same panels. Heat is the enemy.
- Mounting: Roof-flush mounting reduces airflow and increases cell temperature. Standoff mounts (4+ inches) keep panels cooler. Ground-mount with rear airflow is cooler still.
- Quality control during manufacturing: Tier-1 manufacturers have better QC, fewer microcracks during production and shipping. Tier-2 and unknown-brand panels often arrive with embedded microcracks invisible to the eye but eventually limiting output.
What you can do
If you’re buying new:
- Look at the panel’s specific degradation warranty curve, not just “80% at year 25”
- Check independent test reports (PVEL, RETC) for the brand and model
- Prefer half-cut or shingled cells
- Ensure adequate roof airflow
If you have an existing system and want to assess degradation:
- Compare current annual production (kWh/kW installed) to original installation estimate, weather-corrected via PVWatts
- Run IV-curve testing every 5–7 years if you have premium panels worth maintaining
- Thermal imaging (a $300 drone with a thermal camera, or a service company) can identify hot-spot panels before they fail
FAQs
Are degradation rates getting better over time?
Yes, significantly. 2000-era panels degraded at 0.7–1.0%/year. 2025 panels degrade at 0.3–0.5%/year. Mostly thanks to better encapsulants, better cell architectures, and PID-resistant designs.
Can I make a panel degrade slower?
Indirectly. Keep it cool (good airflow), keep it clean (less hot-spot risk), and avoid mechanical stress (don’t walk on panels). The cell-level chemistry is fixed at manufacturing.
Why does my monitoring app say my panels are degrading faster than expected?
Monitoring apps often compare current output to STC-rated output, not real-world expected output. Real-world is always 15–25% below STC due to temperature, soiling, and weather. Check via PVWatts adjusted for your location and weather.
Do degradation rates accelerate near end-of-life?
Slightly. The first 5 years see fastest degradation (LID + early thermal cycling). Years 5–25 are linear. Years 25+ slowly accelerate as encapsulant ages.
What’s a “knee point” in panel degradation?
The point at which degradation suddenly accelerates due to encapsulant failure, delamination, or cell cracking. Some old panels hit this around year 25–30; modern panels probably hit it at year 35–45.
The landing
The 0.5%/year figure isn’t a promise; it’s a median. If you buy premium and treat your panels well, you’ll outperform it. If you buy bargain panels and install them flush against a hot dark roof, you’ll underperform it. The Vermont array Linda tested is just doing what well-built, well-installed panels do — keeping their output up, year after year, mostly being ignored by their owner. The cheapest watt-hour you’ll ever generate is the one your 20-year-old panel produces because it’s still working.