Solar panel warranties and degradation: what is normal?

Solar panels last a long time, but they produce a little less every year. How quickly that happens – and what the manufacturer promises about it – varies from panel to panel. This guide explains how to compare warranties and what degradation is realistic.

Two warranties, two promises

Almost every panel comes with two separate manufacturer warranties:

The performance warranty sounds more impressive, but the product warranty is often the one that matters: a failed panel produces nothing, whereas normal degradation costs only a few per cent.

Separately, your installer’s workmanship guarantee covers the installation itself – roof leaks, cabling, fixings. Ask how long it runs and whether it is insurance-backed in case the company ceases trading.

How fast do panels really degrade?

The most-cited review comes from the US National Renewable Energy Laboratory (NREL), which collected nearly 2,000 measured degradation rates for modules and systems ¹:

• median 0.5% a year, mean 0.8% a year;
• 78% of all results were below 1% a year;
• the authors note that about 0.5% a year is needed to meet a typical 25-year commercial warranty; thin-film modules in their data were more often closer to 1% a year.

Fraunhofer ISE uses 0.5% a year yield degradation in its cost calculations ².

Worked example: what does “80% after 25 years” mean?

A warranty promising 80% of rated power after 25 years equals an average loss of 0.8% a year. A warranty promising 85–90% after 25 or 30 years is a much stronger commitment.

Remaining power with straight-line degradation:

Many warranties allow a larger drop in year one (light-induced degradation) and a fixed annual rate after that. Check both figures on the warranty certificate.

Normal ageing versus a defect

Not every loss of output is normal degradation. Causes that go beyond the usual and may be a warranty claim:

• Light-induced degradation (LID/LeTID): a drop in the first months of operation; normal up to the year-one figure the warranty allows.
• Potential-induced degradation (PID): power loss caused by high system voltage relative to earth, often worst at one end of a string.
• Micro-cracks in cells, from transport, people walking on panels or hail; visible in electroluminescence images.
• Hotspots: individual cells running much hotter than their neighbours, visible with a thermal camera.
• Delamination or discolouration of the encapsulant.

If one string drops noticeably in your monitoring app, or a panel shows up hot on a thermal image, get it documented before you make a claim.

What certification says about lifespan

Panels sold in the UK are normally certified to IEC 61215 (design qualification) and IEC 61730 (safety). IEC 61215 includes thermal cycling, damp heat and mechanical load tests, but the IEC is explicit that test results are not a quantitative prediction of module lifetime ³. A certificate is a minimum bar, not a promise of 30 years.

The Energy Saving Trust says panels should last 25 years or more, while the inverter typically needs replacing after around 12 years ⁴ – budget for that when you compare quotes.

Our data on warranties

To be upfront: our panel data comes from the Californian CEC list, which contains no warranty fields, so we don’t show warranty terms per panel. Always check the manufacturer’s warranty certificate, not just the brochure.

Warranty checklist

1. Product warranty length – and whether it depends on installation by an approved installer or on registering the panels.
2. Performance warranty – starting value after year 1, annual decline, end value.
3. Who pays for removal, scaffolding and reinstallation if a panel fails? These costs are often excluded.
4. UK point of contact – a UK or European office or distributor makes claims far easier. If the manufacturer goes bust, the warranty may be worth little; ask about insurance-backed cover.
5. Inverter warranty – separate and usually shorter; see our inverters.

Compare panels on measured values in our solar panel database.