What is solar panel degradation?
Degradation refers to the gradual decline in a solar panel's maximum power output relative to its rated (nameplate) capacity. A panel rated at 400W that has degraded to 380W is operating at 95% of its original output — a 5% degradation.
The main mechanism is light-induced degradation (LID): the silicon inside the cell is permanently altered by exposure to light in the first days and weeks after installation, causing an immediate 1–3% drop in output that stabilises once the panel has been exposed to full sun for the first time. This is normal and expected, and is why initial output is always slightly below the nameplate rating.
After the initial LID stabilisation, ongoing degradation continues at a much slower rate driven by changes to the silver contact fingers (which corrode slowly), delamination of the ethylene-vinyl acetate (EVA) encapsulant that surrounds the cells, and yellowing or browning of the backsheet. Temperature cycling — the daily expansion and contraction of materials as panels heat and cool — also causes microcracks in cells over time.
What the numbers look like in practice
The industry standard degradation rate is approximately 0.5–0.7% per year for monocrystalline silicon panels, which are the dominant type in UK domestic installations since around 2018. Most manufacturer performance warranties guarantee no more than 20% degradation over 25 years — i.e. that the panel will produce at least 80% of its rated output at year 25.
At 0.6%/year, a 400W panel that produced 395W in year one (after initial LID) will produce approximately 368W at year 10 and 349W at year 20. For a 4 kWp installation (10 × 400W panels), this means generation falling from roughly 3,400 kWh in year one to about 3,100 kWh by year 20 — assuming no other degradation factors intervene.
Premium panel brands (SunPower, REC Alpha, Panasonic) achieve degradation rates below 0.4%/year. Budget panels often carry a 30-year performance warranty clause that explicitly permits faster degradation in the first decade — always read the performance warranty, not just the product warranty period.
What accelerates degradation
High operating temperatures significantly accelerate degradation. Panels mounted flush against a roof surface with no air gap underneath run hotter than panels on a ballasted flat-roof array with full ventilation. Thermal stress compounds over years. In the UK's temperate climate this is less severe than in southern Europe, but roof-integrated panels (BIPV) with no rear ventilation degrade faster than conventionally mounted panels.
Physical stress events — a roof tile falling onto the array, heavy snow load, a clumsy maintenance worker stepping on a panel — can cause microcracks that accelerate localised degradation far beyond the normal curve. This is one reason why professional cleaners who are trained not to stand on panels matter.
Prolonged soiling — particularly bird droppings or moss that retains moisture — creates localised hotspots where trapped moisture and electrical current interact to corrode cell contacts and delaminate encapsulant. This is a form of accelerated localised degradation, distinct from the uniform gradual decline caused by ageing.
How to slow it down
Annual professional cleaning removes soiling that would otherwise cause hotspot-driven localised degradation. Ensuring the rear of the array has adequate ventilation — which is determined at installation but worth knowing about if you are ever re-roofing — reduces operating temperatures.
A visual inspection of the panel surfaces from the ground each year, looking for obvious physical damage, discolouration, or delamination blistering visible through the glass, gives early warning of panels that may be degrading faster than the norm.
If your monitoring data shows a panel or panels tracking consistently below the rest of the array without obvious soiling explanation, an infrared thermal inspection can identify hotspots or bypass diode failures not visible to the naked eye. This is typically offered by specialist solar maintenance companies rather than general cleaners.