Solar Panel Degradation Rates

Every solar panel loses a small fraction of its power each year, a slow fade called degradation. For modern panels the rate is about 0.4 to 0.55 percent annually, which compounds to roughly 12 percent over 25 years. Understanding why panels degrade, and how to read the degradation curve buried in your warranty, lets you predict lifetime output and spot the difference between a good panel and a great one.

Reading a degradation curve

Every panel datasheet includes a performance warranty chart: output percentage on the vertical axis, years on the horizontal, with a guaranteed-minimum line sloping gently downward. A typical mainstream warranty promises at least 98 percent after year one, then a linear decline to about 87 percent at year 25. A premium warranty holds 92 percent at year 25.

Two things about that line. First, it is a floor, not a prediction. Manufacturers set it where they are confident nearly all panels will stay above it; typical panels perform somewhat better than the line. Second, the slope is the number that matters: 0.5 percent per year versus 0.25 percent per year looks trivial on paper and compounds into a real gap, as the math below shows.

Our panel lifespan guide covers what these curves mean for total years of service; this article is about the physics behind the slope.

The first-year drop: LID explained

Most panels lose 1 to 2 percent of output in their first days of sunlight exposure, then settle into the slow annual fade. The main culprit is light-induced degradation. In P-type (boron-doped) silicon, the first photons trigger formation of boron-oxygen complexes that permanently trap some charge carriers. The panel stabilizes within days, but those percentage points never return.

Manufacturers know this and rate panels post-LID, so you are not being cheated; the day-one rating already accounts for it. But LID explains why year-one production often lands slightly below the installer’s model, and why N-type panels, which use phosphorus doping that does not form these complexes, show first-year drops closer to 1 percent. It is the first of several reasons N-type cells age more gracefully.

The long annual fade

After year one, degradation settles into its long rhythm, driven by several slow mechanisms:

• Encapsulant browning. The EVA plastic sealing the cells gradually yellows under decades of UV, letting slightly less light through. Better encapsulants and glass-glass construction slow this.
• Cell micro-cracks. Thermal cycling, flexing in wind, and installation handling create tiny cracks that slowly isolate cell fragments, trimming output.
• Corrosion. Moisture ingress, however slight, corrodes the silver fingers and busbars carrying current. Good edge seals and dry climates help.
• Backsheet aging. UV and heat gradually degrade the backsheet, in rare cases cracking and admitting moisture that accelerates everything above.

Field data from NREL’s long-term studies puts the median real-world rate near 0.5 percent per year across many system vintages, with modern quality panels clustering at 0.4 percent and premium N-type at 0.25 to 0.3 percent. These are measured medians from thousands of systems, not lab claims. NREL’s photovoltaic reliability research and the Department of Energy’s Solar Energy Technologies Office both publish accessible summaries of this degradation work for non-specialists.

PID and LeTID: the acronyms that matter

Potential-induced degradation (PID) occurs when high system voltage drives sodium ions from the glass into the cells, shunting power away. It hit some early-2010s systems hard, with losses of 30 percent or more in bad cases. Modern panels are tested to PID-resistant standards (IEC 62804), transformerless inverters reduce the driving voltage, and proper grounding finishes the job. On a 2026 install with reputable equipment, PID is a solved problem, but it is worth knowing the term in case you inherit an old system showing mysterious underperformance.

Light- and elevated-temperature-induced degradation (LeTID) is the subtler successor. It affects some PERC cells exposed to heat plus light over months, causing a few percent of reversible-plus-permanent loss. Manufacturers have reformulated cell processes to suppress it, and it rarely troubles modern panels. If your datasheet mentions LeTID testing, that is a good sign, not a warning.

Why N-type degrades slower

The chemistry is straightforward: phosphorus-doped N-type silicon avoids the boron-oxygen defects behind LID, and the passivation layers in TOPCon and HJT designs better protect cell surfaces over time. The result is a flatter curve that you can see in every premium warranty.

Put dollars on it. An 8 kW system making 12,000 kWh in year one at $0.20/kWh: at 0.6 percent annual degradation, 25-year lifetime production is about 278,000 kWh. At 0.3 percent, it is about 288,000 kWh. The 10,000 kWh difference is worth roughly $2,000 at current rates, more as rates climb. That is the lifetime value of the flatter curve, and it is why degradation rate belongs in every equipment decision alongside price.

Using degradation math as a buyer

1. Read the warranty curve, not the marketing. Find the year-25 guaranteed percentage in the warranty document. Divide the gap from 100 by 25 for the effective annual rate.
2. Model with the warrantied rate. When comparing quotes, apply each panel’s warrantied degradation to 25-year production. The cheapest quote with fast-degrading panels can lose on lifetime cost per kWh.
3. Watch for step warranties. Some warranties guarantee 90 percent for 10 years then drop faster. Linear warranties are easier to compare and generally stronger.
4. Do not overthink small differences. The gap between 0.4 and 0.5 percent is worth maybe $500 over 25 years. The gap between 0.3 and 0.7 percent is worth thousands. Focus on big deltas.
5. Monitor to verify. Panel-level monitoring lets you confirm your system tracks its expected curve. A system degrading faster than warranty is a claim worth filing.

One more practical use of this math: evaluating used or inherited systems. If you buy a home with 10-year-old panels, apply the warrantied degradation rate to estimate remaining output. A 2016 system with mainstream panels has likely lost 5 to 8 percent of its original production, which should be reflected in any production estimate the seller provides. Ask for a year of monitoring data rather than trusting nameplate ratings; the data shows the degraded reality, which is what you are actually buying.

Degradation is the quiet variable in solar economics: invisible day to day, decisive over decades. A few minutes with the warranty curve tells you more about lifetime value than most of the sales pitch. And because the curve compounds, the small annual differences between panel tiers deserve more weight in your decision than their modest size suggests.