Design, installation and performance
Degradation
Also known as: performance loss, annual degradation.
Definition
Degradation is the gradual loss of panel capacity over the years, typically around 0.5% a year. That is why performance warranties guarantee a minimum percentage of output at 25 years.
The decline is not uniform. In the first year there is a somewhat larger initial degradation, linked to the first exposure to light, and from then on the rate settles into a small, sustained annual loss. A panel that starts at 550 Wp does not become useless at 25 years: it keeps producing, at a capacity on the order of 85-90% of the original depending on each manufacturer's warranty.
The performance warranty is different from the product warranty. The product warranty covers manufacturing defects for a set period; the performance warranty commits to a minimum output curve over 25 or 30 years. Both should appear in writing in the quote, along with the procedure for claiming and who handles it.
Normal degradation has to be told apart from avoidable failures. A module with microcracks from careless mounting, delamination from poor quality or hot spots from permanent shading is not degrading: it is failing. Degradation is predictable and is on the datasheet; failures are not.
The essentials of Degradation
- Typical rate: around 0.5% a year after the first year.
- The first year concentrates a somewhat larger initial degradation.
- At year 25 a panel usually retains close to 85-90% of its capacity.
- The performance warranty is different from the product warranty: ask for both in writing.
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Frequently asked questions about Degradation
Do panels stop working after 25 years?
No. The 25 years are the horizon of the performance warranty, not an expiry date. After that period the panel keeps generating, with slightly less capacity. What should be planned for within that period is the inverter replacement, whose service life is shorter.
How do I know whether a panel is degrading more than normal?
By comparing real output against expected output through the monitoring system. A gradual, gentle decline is normal degradation; an abrupt drop, or one localised to a few modules, points to a fault, dirt or new shading, and deserves a technical inspection.
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