Picked up a 200W panel from Facebook Marketplace – worth it or have I been done?

Just picked this up for £35 from a bloke about 20 miles away — no brand markings I can recognise, just a sticker saying "200W 24V" and what looks like it might be a Chinese OEM panel from about 5 years ago. Glass is clean, no visible delamination or cracked cells when I hold it up to the light.

Ran a quick open-circuit voltage test in the garden and got around 21V in patchy cloud, which seems roughly plausible? Haven't been able to do a proper full-sun test yet because, well, it's Britain.

A few things I'm not sure about:

• Is there a reliable way to test actual wattage output without specialist kit?
• Could the cells be degraded enough to make it essentially useless even if it looks fine?
• Anyone know if mismatched panels cause serious issues when wired alongside a known-good Renogy 200W I already have?

I've got a Victron SmartSolar 100/30 so at least I can monitor what it's actually pulling through the app — wondering if that gives me enough data to work out whether it's performing anywhere near spec.

Part of me thinks even if it's only putting out 120-130W realistically, for £35 it's still decent value. But if it's going to cause problems for my existing setup I'd rather know before I wire it in permanently.

Has anyone else taken a punt on second-hand panels from Marketplace or Gumtree? What should I actually be looking for before committing to using it properly?

@AlanPalmer57 "no brand markings I can recognise" is doing a lot of heavy lifting in that sentence 😂

Genuine question though — have you actually stuck a multimeter on it yet? Because "200W 24V sticker" and "200W 24V panel" are two very different things in the world of Facebook Marketplace physics

@AlanPalmer57 Stick a multimeter on it in decent sunlight — if you're seeing anywhere near the claimed Voc (should be ~37-38V open circuit for a "24V" panel) it's probably not complete rubbish, but "probably not complete rubbish" is the best you're getting for £35 off Facebook.

Got three mystery panels on my static van myself, two of them are genuinely fine, one of them I'm fairly convinced is generating electricity purely out of spite.

Run it through a proper MPPT controller (not whatever PWM tat might have come with it) and check actual wattage output — a Victron SmartSolar will tell you exactly what you're working with. If it's doing 150W+ on a clear day you've had a result. If it's doing 40W, well. Lesson learned for £35.

@AlanPalman57 £35 for a 200W panel is either a bargain or a bin lid with ambitions — the multimeter test @NotAnElectrician80 mentioned will tell you quickly enough, but I'd also check the junction box on the back. If the diodes look like they've been soldered in by someone's nan during a power cut, that's your first red flag.

Also worth checking the glass for micro-cracks — run your hand across it in raking light at an angle. Delamination round the edges is another giveaway it's been stored badly.

What's your intended use? For a static caravan or garden office with a decent MPPT it might still be perfectly usable even if it's only putting out 140-150W in practice. Degraded ≠ dead.

Beyond the basic multimeter check @NotAnElectrician80 already mentioned, I'd also do a shading test — cover roughly a third of the cells with a piece of cardboard and watch what the current does. Dodgy panels with failed bypass diodes will drop catastrophically rather than proportionally. Had a no-name 150W from a car boot that passed the open-circuit voltage test fine but was basically useless the moment anything caused partial shading — which in the UK is, well, constantly.

Also worth checking the junction box on the rear. If the diodes in there look corroded or the sealant is cracking, moisture has likely got in at some point and cell degradation follows.

£35 is low enough risk that even 60-70% of rated output isn't a disaster for a static caravan setup, but go in with realistic expectations rather than the sticker figure.