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

Definitely worth a check before you commit to wiring it in properly. £40 is a decent score but used panels can be hiding all sorts.

First thing I'd do is a visual inspection in decent light — look for microcracks, hotspot discolouration (brownish patches on individual cells), or delamination at the edges where moisture can get in. If the encapsulant's gone yellow or bubbly, that's usually a sign it's been baking in the sun for years without much care.

Beyond visual, if you've got a multimeter you can at least check open-circuit voltage on a clear day and compare it against the spec sheet. Should be close to the rated Voc. A significant drop suggests cell damage. Ideally you'd want a clamp meter to check Isc as well, but not everyone has one lying around.

I picked up a secondhand 175W panel last year from a local farm clearance — looked fine on the surface but one string of cells was barely contributing. Ended up getting maybe 60% of expected output. Sold it on cheap and learned my lesson about skipping the checks.

If you've got access to a thermal camera (some hire shops do them, or ask around locally) that's the gold standard for spotting hotspots under load. Not essential for a £40 punt but worth knowing the option exists.

What inverter or charge controller are you running it into? If it's going through a Victron MPPT you'll get decent yield data fairly quickly and can see if something's off.

Anyone else on here got a go-to method for quickly vetting secondhand panels?

@Nessa73 grab a thermal camera on a sunny day and any dead cells will light up like a dodgy Christmas tree — Flir have a decent phone attachment that won't bankrupt you.

@DaleLover the thermal camera tip is gold but if you haven't got one handy, there's a dead simple trick I stumbled onto when I picked up three secondhand panels for my shepherd's hut build — grab a clamp meter and check the short-circuit current (Isc) panel by panel in full sun. Your 200W panel should be pushing somewhere around 8-10A depending on the spec sheet. Anything significantly lower and you've got dodgy cells dragging the whole string down.

Also worth checking the junction box at the back — peel back the seal slightly if it looks weathered. Mine had water ingress that wasn't visible until I actually looked properly. Killed the bypass diodes completely.

£40 is a cracking deal if it checks out, genuinely. Could be a solid addition to someone's garden office setup if the numbers stack up.

@VivaroNomad go on then, don't leave us hanging!

From my narrowboat days I learned the hard way that a dodgy panel can drag down your whole string. Had a 175W panel that looked immaculate but was putting out barely 60W on a clear day.

What actually saved me was just doing a proper open-circuit voltage test first — dead cells will show up in a noticeably low Voc reading compared to the spec sheet. Then I ran an Isc test. Between those two numbers you get a pretty clear picture before you've drilled a single cable gland.

For £40 it's worth spending an afternoon properly characterising it rather than wiring it in blind and wondering six months later why your Victron MPPT is behaving oddly.

@VivaroNomad yes please finish that thought!

One thing I'd add — before any of that, just chuck a cheap clamp meter on it in full sun and compare the actual Isc against the spec sheet. My 200W Renogy panel should be doing around 9.6A in decent conditions, so if a used one comes in at 7A or below on a clear day, something's clearly off.

Also worth checking the junction box on the back — diodes fail silently and a blown bypass diode means you're losing a whole cell string without anything obvious showing visually. You can test them with a basic multimeter on diode mode, takes about two minutes.

For £40 the panel might still be fine even with one duff diode, replacements are pennies. Just factor it into whether the price is actually a bargain once you've done the work.

@VivaroNomad seconding everyone else — finish the thought!

One test I swear by before wiring anything permanently: EL imaging isn't accessible to most of us, but you can approximate cell-level fault detection by shade-testing individual rows with a piece of cardboard while monitoring current with a clamp meter. Disproportionate current drop when shading a single row often points to a dud bypass diode or cracked cells underneath.

Also worth checking the junction box on the back — peel back the lid and inspect the bypass diodes and solder joints. On cheap or aged panels they corrode badly in UK weather, and a failed diode will tank your output under any partial shading. Replacements are pennies on eBay.

£40 is absolutely worth spending an hour diagnosing properly before committing to cable runs.