Solar panel degradation: how much output you lose per year

Modern panels lose about 0.5% output per year. After 25 years, they still produce 87-88% of rated power. Here's what actually causes the loss and what you can do about it.

Solar panels don't stop working after their warranty expires. They don't suddenly fail at year 25. What they do is gradually, predictably lose a small fraction of their output each year. Understanding the rate and cause of this degradation matters for sizing your system correctly, planning battery replacement cycles, and setting realistic expectations for long-term production.

The numbers

According to NREL (National Renewable Energy Laboratory) data from hundreds of real-world installations, the median degradation rate for modern crystalline silicon panels is about 0.5% per year. That means a panel rated at 400W produces about 398W in year two, 396W in year three, and so on.

After 25 years at 0.5% annual degradation, a panel retains approximately 88% of its original rated output. A 400W panel produces about 350W. That's still a useful, productive panel. It didn't break. It just got a bit slower.

Premium monocrystalline panels from top-tier manufacturers (SunPower, REC, Panasonic) degrade at 0.25-0.35% per year. At 0.3% annual degradation, a panel retains about 93% of original output at year 25. Older polycrystalline panels and budget brands may degrade at 0.7-1.0% per year, retaining only about 78-84% at year 25.

Newer N-type cell technologies (TOPCon, HJT) show even lower degradation rates in early field data, partly because they're immune to the light-induced degradation that affects traditional P-type cells.

What causes degradation

Light-induced degradation (LID)

LID is a one-time performance drop of 1-3% that occurs in the first hours to days of sunlight exposure. It's caused by a reaction between boron (the dopant in standard P-type silicon cells) and oxygen in the crystal lattice. The reaction creates defects that reduce the cell's ability to convert light into electricity.

LID happens once and doesn't continue. Most manufacturers account for it in their first-year warranty terms (guaranteeing 97-98% of rated power after year one rather than 100%). If your panel's first-year output is 2% below the nameplate rating, that's LID, not a defect.

N-type cells (used in TOPCon and HJT panels) use phosphorus instead of boron as the dopant and are immune to LID entirely. This is one of the reasons premium panels have lower total degradation over their lifetime.

Potential-induced degradation (PID)

PID is caused by voltage stress between the solar cells and the grounded frame, particularly in humid environments. High system voltage (common in grid-tied string inverter setups with many panels in series) drives ion migration through the encapsulant material, which reduces cell performance.

PID can be severe (10-30% loss in affected panels) but is largely a concern for large grid-tied installations, not small off-grid systems. Off-grid systems with twelve-volt or twenty-four-volt configurations have much lower voltage stress and minimal PID risk. Panels certified to IEC 62804 anti-PID standards are resistant.

Thermal cycling and mechanical stress

Daily temperature swings cause the materials in a solar panel (glass, silicon, aluminum frame, solder joints, polymer backsheet) to expand and contract at different rates. Over thousands of cycles, this creates micro-cracks in the silicon cells and fatigue in solder connections. Micro-cracks reduce the active cell area and increase resistance, both of which reduce output.

Panels in climates with large daily temperature swings (desert environments where panels see 70 C during the day and 10 C at night) experience more thermal cycling stress than panels in mild, stable climates.

Mechanical stress from wind loading, snow loads, and installation handling also creates micro-cracks. Panels that are properly mounted and not flexed during installation show less mechanical degradation than poorly handled or incorrectly racked panels.

UV degradation of encapsulant

The polymer encapsulant (EVA or POE) that seals the cells between the glass and backsheet slowly yellows under UV exposure. Yellowed encapsulant transmits less light to the cells, reducing output. This is a gradual effect that contributes to the ongoing annual degradation rate.

Modern encapsulants are formulated with UV stabilizers that slow this process, but all polymer materials eventually degrade under decades of UV exposure.

Moisture ingress

If the panel's edge seals or backsheet develop micro-perforations (from UV degradation, thermal cycling, or manufacturing defects), moisture enters the panel. Moisture accelerates corrosion of the cell metallization and interconnects, increases PID risk, and can cause delamination of the encapsulant from the glass or cells.

Moisture-related degradation is the primary mechanism behind the higher degradation rates observed in hot, humid climates compared to cool, dry climates.

What you can control

You can't stop degradation, but you can minimize accelerating factors:

Keep panels clean. Dirt, bird droppings, pollen, and leaf debris reduce light reaching the cells and can create hot spots (localized overheating) that accelerate cell degradation. Clean panels with water (no abrasive cleaners, no pressure washers) once or twice a year, or more often in dusty environments.

Ensure adequate ventilation behind panels. Panels mounted flat against a roof with no air gap run hotter than panels with 10-15 cm of airflow space behind them. Higher operating temperatures increase degradation rate. Racking systems that allow air circulation behind the panel extend its life.

Proper mounting and handling. Don't flex panels during installation. Don't walk on them. Don't overtighten mounting clamps. Micro-cracks from mechanical stress are invisible but cumulative.

Monitor output over time. Track your system's daily or monthly production. A sudden drop in output (not explained by weather or seasonal changes) may indicate a developing problem (a failed bypass diode, severe PID, or a wiring issue) rather than normal degradation. Normal degradation is gradual and barely noticeable year to year.

What this means for system sizing

If you're designing an off-grid system today that needs to power a load for the next 15-20 years, factor degradation into your sizing. A system sized to exactly meet your load at year zero will be undersized by 7-10% at year 15.

The standard approach is to add 10-15% to your calculated panel capacity to account for degradation and real-world losses (soiling, shading, temperature derating). A system designed to produce 110% of your current needs will still produce 100% of your needs at year 15, even with normal degradation.

The solar array calculator helps you size panel capacity with these factors included.