Some large PVs for rooftops were at a street market for €35 each. I’m not deeply knowledgable about them… I just know that there are two varieties of solar panels and that the kind that are used from small appliances (e.g. calculators, speakers, lawn lights, etc) are junk. And that junk variety is sometimes used in large rooftop panels. What I was looking at resembled the kind I see on a bluetooth speaker with a slight blue tint so I was skeptical. The info on the backside of the panel indicated “1000 V”. The other thing is, all solar panels degrade over time and reach end of life after like 15 years (though this is improving). They may have been a good deal but I passed on them because I didn’t want to buy them on a blind risk.
How would I know how much life a used PV has left? Would a volt meter give that info, assuming it’s sunny when I encounter them again?
The absolute cheapest 420Wp panels are €50-80 new. That would be a roughly 1.7m X 1.1m panel. So 35€ for the risk of buying scrap and them having been degraded would be a bad deal. If you can get the rest of the needed installation parts with it, it might be worth it or if those are new panels.
If you want to get started there are solar home kits online, which are rather easy to install, if you have some very basic diy skills. The electronics is basically plugin in some cables and the difficult part would be actually setting up the panels. On a flat roof that might just be screwing on some metal stands and drilling a hole to run the cables through.