Originally Posted by SherpaDave
Just keep them clean. Any dust kills efficiency so hopefully they aren't on your roof. Also keep in mind they lose about 5% efficiency per year. Maybe yours are a little better but that is about average. That means they will only produce about half at your payback period.
0.5%, that's zero point five percent per year.
5% per year is a warranty claim. Also, they don't actually degrade in a linear fashion, but the average over a number of years usually works out to be about .5% or less. You are actually likely to get more performance degradation from loose connections or corrosion at terminals. It's a good practice to check all of the connections once a year (without zapping yourself).
Degradation is an impediment to the flow of electrons. It is mostly a factor of manufacturing methods and how the modules were handled during shipping and installation. Undue stresses on the modules cause microscopic cracks to form at the cell junctions and these grow during heavy freeze/thaw cycles or extreme vibration from wind. That's why I said that degradation is event driven and not linear.
There are a lot of modules that may have the exact same label on them but were actually produced in different factories using different methods. Type "A" may experience almost no degradation and Type "B" may be right at or over the warranty threshold. There's no practical way to know unless the modules were tested using very expensive equipment that is really only practical in large utility installations.
The reality in residential applications is that most people quickly stop paying attention to their solar after the "new" wears off and have no idea how well their system is performing. It just sits there until someone notices the red light that tells you that the inverter is faulted. It's the "small" performance issues caused by soiling, shading, loose or corroded connections, etc. that really affect the payback period. It is also advisable to set aside a rainy day fund because the inverter is almost certainly only designed to last 10 years, and replacing that will obviously change the payback calculation as well.