How old were the panels? I understand as they age, the output drops. Have you noticed any effects?
His panels were made in 1994 and we bought them in 2004. So we can't tell if their output is dropping, even if they were on their own and not part of the other panels (of three other models) in the two strings. Half of our array's panels are from the 1990s and the other half are manufacturer's rejects from 2005.
The only reason the 80-year-old was selling the panels was because he'd broken his hip. It was healing but he was having trouble getting around to take care of his batteries, which he'd essentially beaten to crap (the batteries, not his hip) by his off-the-grid max-deep-cycle behavior. He was charging the batteries at day and then running them down at night (until the voltage regulator tripped off-line) and not adequately watering them. So he thought the system was worn out and we got the panels for not much more than the labor of removing them.
I've looked into checking rated output but there are too many variables. For example our generation this year has sucked (I expected 325 KWHr/month and we've been getting ~300) because of cloudy weather (rain in July?!?) and vog. Our inverter manual suggests checking production at local noon on 21 March & 21 September-- the solstices-- but even then clouds or windy weather (panel temperature) can overwhelm the attempt to hold conditions constant by putting the sun in the same position. The only way to truly verify output would be to isolate the panel from the string, put it into a lightbox of known lumens, and check the panel's power. Only a research lab would go to that effort.
Like buying a used car, the resale value of a used panel drops far more quickly than its functional value! Technology helps, too-- manufacturers generally offer a 25-year warranty, but every technical improvement knocks down the price of the previous generation's panels. Today's most impressive improvements aren't so much in efficiency as they are in production. Instead of growing crystals on cells the cells are starting to be "printed out" by thin-film techniques. That lets arrays be built to almost any size, configuration, or application. One solar installer at a local home show last weekend had laid the 12" squares of his plastic-wrapped rubber-backed thin-film panel string out like a sidewalk, and he enjoyed watching people freak out when he told them they were walking on the array.
I don't know anyone who's replaced their panels because of age. More power, higher power density, storm damage, a new roof, or even better decor-- sure. But not age.
