These were poly, from Sharp. My other panels of the same vintage are mono, from AstroPower.
Sharp's warranty is 90% for 10 years, 80% for 15 more years. I haven't tried the warranty service yet, but another forum member indicated this model was being replaced by Sharp. So I'm hoping that is entire array replaced with panels that don't have this degradation mechanism.
Previously I thought the Sharps were holding out well, producing near 100%, but AstroPower showed early degradation.
I then ran both (and SunPower) on a transformerless GT PV inverter for a year, which biases half the panels negative and half positive.
Previously all were positive biased (negative ground array.)
Then, I moved all panels to another model inverter which can be jumpered for either positive or negative ground. I positive grounded the SunPower panels, negative grounded the others.
Once I bought a clamp DC ammeter, I checked all strings. One SunPower was open circuit due to wire nut, which I fixed. One Sharp string was half the current of the other, which is why I did that testing. Problem was 5 panels under producing, all in one string. IR images seemed to show some hot cells but not as severe in other panels.
What I'm not sure of is if this degradation was occurring over time, or if it happened during the year of living negative biased. PID (potential induced degradation) has some reversible and some irreversible mechanisms. Maybe that happened.
You can't necessarily count on young panels being in good shape. It varies by brand/model, and environment. Hot and wet is one of the conditions which causes some kinds of damage. Some panels are more susceptible than others.
Well that sounds like a heck of a deal! Glad you placed your order and can't wait to see how it all works. That is a very good price, and I should not be surprised but that confirms they get a great deal when they upgrade. I wonder if they went with Trina on the replacement units? Any idea?
diysolarforum.com
That was a colleague's fancy $600 thermal imager, so a one-time thing. That, a clamp ammeter, remote data from micro inverters, historical data from SCC or string inverters would high differences from neighbors or previous performance. I finally have a logger connected, but haven't bothered to look at it. I just look at monthly net-metering bills.