Picked up a used 200W panel for £40 — worth checking the cells before wiring it in?

Absolutely worth checking — learned this the hard way about three years ago when I picked up a "bargain" 250W panel from a van conversion lad who'd clearly left it face-down on gravel for a season.

Here's what I'd do before wiring anything in:

1. Visual inspection first — get a torch and look for microcracks, delamination, or any browning around individual cells. Moisture ingress shows as a faint misting between the glass and backsheet.
2. Voc test with a multimeter — in decent midday sun, you should be hitting close to the rated open-circuit voltage. Significantly lower and you've got dead or shunted cells.
3. Check the junction box — bypass diodes fail silently and you'd never know until half your panel stops contributing under partial shade.

A £40 panel can be brilliant value, even at 60-70% original output it's still usable for battery top-up or a secondary emergency backup circuit. My workshop runs off two panels in exactly that condition — both Renogy frames with replacement cells, wired through a Victron MPPT that doesn't care they're mismatched.

The real risk isn't reduced output, it's a compromised cell creating a hot spot under load. That's where panels start fires, and I've seen it happen to a mate's narrowboat setup.

Worth running it in isolation for a week before integrating it into your main array.

What condition's the frame and junction box in? That'll tell us a lot about how hard a life it's had.

@TorJake that gravel story made me wince — I've seen that exact damage pattern on panels pulled from old boat roofs too.

One thing worth adding: grab a cheap clamp meter and test the Isc (short-circuit current) on a clear day around solar noon. Compare it against the panel's spec label. If you're getting less than 85% of rated current, cells are likely degraded or there's delamination letting moisture in.

I picked up a scratched 200W panel off eBay for my narrowboat last spring — looked rough but tested at 94% of spec. Still running it now.

The real killers are microcracks, which won't show up visually. If you can borrow a thermal camera during operation, hotspots will show you exactly which cells are struggling before you commit it to your system.

@TorJake £40 for 200W is genuinely decent if the cells are intact — that's the critical caveat nobody mentions enough.

Beyond the obvious physical inspection, grab a cheap thermal camera app or borrow a proper FLIR if you can. Hot spots show up beautifully under load on a sunny day — dead or cracked cells will glow like a Christmas fairy light compared to their neighbours. I caught a nasty bypass diode failure on a secondhand Renogy panel this way before it caused me any real grief.

Also worth doing a quick IV curve trace if you've got a decent charge controller with logging — a healthy 200W mono should hit close to its Voc spec even before you stress it. Significant deviation tells you cells are gone.

Delamination is the other silent killer — press gently around the frame edges and listen for that hollow crinkle.

@TracyAllen how do you actually go about checking the cells properly though? I've got a cheap multimeter but I'm never quite sure what figures I should be seeing on a second-hand panel — like, is there a meaningful difference between checking it in full sun versus overcast?

Mine's destined for the static caravan setup so I want to be confident before I wire it into the Victron MPPT. I've heard thermal imaging can reveal cracked cells that look fine visually, but that feels a bit overkill for a £40 panel — or is it actually worth hunting down someone with a thermal camera?

Also wondering whether partial shading from a single dodgy cell would just drag down the whole string or if bypass diodes handle that gracefully on most panels?

@ThingamyBob a multimeter's fine for a quick sanity check (Voc and Isc in direct sun, compare against the label), but the real trick is a thermal inspection on a bright day — hotspots show up like a Christmas tree and that's where your dodgy cells are hiding; failing that, shine a torch through the back in a darkened space and cracked cells will reveal themselves like a sad stained-glass window.

@FETFan is right about the thermal imager being the gold standard, but there's a middle-ground option worth knowing about: EL (electroluminescence) testing. Proper kit for that is pricey, obviously, but some solar installers will run a panel through one for a small fee if you ask nicely.

For a £40 gamble though, I'd start with the visual basics that a multimeter simply can't tell you — get it flat on the ground in decent light and look for:

• Micro-cracks: faint spider-web lines across cells
• Delamination: cloudy or bubbling areas under the glass
• Hotspot discolouration: brownish or yellowed patches on individual cells

My shepherd's hut setup has two "character" panels I rescued from similar situations. One's been fine for four years. The other developed a hotspot I wish I'd spotted before wiring it into my Victron MPPT.