Panels age slowly, not suddenly
Solar panels have no moving parts, but they do not last forever at full strength. Over years of sunlight, heat, cold and moisture, their output gradually declines. This slow loss is called degradation. A healthy panel does not stop working at the end of its warranty; it simply produces somewhat less than it did when new. Understanding why this happens makes it easier to compare panels and to read the warranty that comes with them.
The main causes of degradation
Light-induced degradation (LID)
Some cells lose a small amount of output during their first exposure to sunlight, caused by reactions involving impurities in the silicon. This mostly affects older p-type cells. It is one reason datasheets often guarantee a lower output for the first year than the subsequent yearly rate would suggest.
Light and elevated temperature induced degradation (LeTID)
A related effect can appear over a longer period when panels run hot in strong sunlight. Manufacturers have developed processing steps to reduce it, and n-type cell technologies are generally less susceptible.
Potential-induced degradation (PID)
In large systems, a voltage difference between cells and the grounded frame can drive tiny leakage currents that reduce performance, especially in hot, humid conditions. Modern panels are often tested and marketed as "PID resistant", and system design can also reduce the risk.
Physical and environmental wear
Thermal cycling (heating by day, cooling at night) stresses solder joints and can cause microcracks in cells. Moisture can slowly corrode contacts if it gets past the encapsulant. UV exposure can discolour encapsulant materials. Hail, heavy snow loads and poor handling during installation can create cracks that grow over time.
What a performance warranty says
Most panels come with two separate warranties:
- Product (materials and workmanship) warranty: covers defects in the panel itself, such as a failed junction box or delamination.
- Performance (power output) warranty: guarantees that the panel will still produce at least a stated percentage of its rated power after a stated number of years.
A performance warranty is usually expressed as a first-year limit plus a maximum annual decline after that, ending in a guaranteed minimum at the end of the term. Commonly cited figures for modern panels are in the region of half a percent per year, but the only numbers that matter for your system are those printed on your panel's datasheet and warranty document. Compare the guaranteed output in the final warranty year when you look at quotes; it summarises the whole curve in one number.
Reading the fine print
- Who pays for labour and shipping? Some warranties replace the panel but not the cost of removing and reinstalling it.
- How is a claim proven? Warranties typically require testing under specific conditions. Monitoring data helps you notice a problem, but it may not be enough to prove a claim on its own.
- Is the manufacturer likely to be around? A long warranty is only as good as the company standing behind it. Some installers offer their own workmanship warranties as well.
- Is registration required? Some extended terms only apply if the system is registered with the manufacturer.
Reducing degradation in practice
- Choose a competent installer who handles panels carefully and follows the manufacturer's mounting instructions.
- Allow airflow under panels; cooler panels age more gently and produce more.
- Use monitoring to spot sudden drops, which usually point to a fault rather than normal ageing.
- After severe hail or storms, have the array inspected rather than climbing up yourself.
Planning with degradation in mind
When you estimate long-term savings, a realistic projection assumes production declines a little each year rather than staying flat. A good installer's estimate should state what degradation rate it assumes. If it does not, ask.
General information only. Warranty terms differ by manufacturer and product; read the actual documents before buying.