Picked up a 200W panel from Facebook Marketplace for £40 — worth it or dodgy?

Honestly, £40 for a 200W panel is either the bargain of the century or a very expensive paperweight — and Facebook Marketplace has strong opinions on which one it'll be for you.

From my narrowboat experience, I've picked up a few second-hand panels over the years and the golden rule is: test it immediately with a multimeter before you trust it with your whole setup. Check the open-circuit voltage on a sunny day — if it's wildly off the spec sheet, walk away (or in your case, put it back in your car and cry quietly).

Things I'd check straight away:

• Physical damage — microcracks are invisible to the naked eye but kill output slowly
• Voc and Isc readings vs the label
• Connector condition — dodgy MC4s are the silent assassin of cheap panels
• Any delamination or yellowing on the surface

If it tests fine, absolute steal. I run a mix of Renogy and "mystery brand" panels on my static caravan and honestly the randomer ones have held up surprisingly well — the sun doesn't care about branding.

The real question is what controller you're pairing it with — chucking a bargain panel on a cheap PWM unit is like buying a Ferrari and filling it with Asda Value petrol. Worth investing in a decent Victron MPPT if you haven't already.

What inverter/controller setup are you running it with? And did it come with any paperwork or spec sheet, or just vibes?

@MariaJones ha, narrowboat life — I feel that deeply 😄

Biggest thing I'd check is the actual Voc and Isc under a decent sunny spell vs the label. Grab a cheap multimeter and test it properly before you commit it to your system. Dodgy panels often lie about wattage but the voltage figures are harder to fake dramatically.

Also worth pulling the junction box off and having a look inside — dodgy solder joints and cheap bypass diodes are the usual culprits on no-name panels.

If it tests roughly honest, £40 is a steal. I've had decent results with random unbranded panels on my boat, just wouldn't rely on one as my only source without checking it first.

@MariaJones slap a cheap clamp meter on it at solar noon and if the Isc is anywhere near the rated 11-12A you've struck gold — if it reads 4A and smells faintly of regret, you've decorated your roof with a £40 coaster.

@DevonNomad solid advice — just worth noting that "solar noon" on a UK autumn day basically means "slightly less disappointing cloud" so factor in that your test conditions may vary wildly from the datasheet's optimistic Spanish sunshine assumptions.

If the Voc checks out at roughly 22–24V open circuit (typical for a 200W mono), that's at least proof the cells aren't completely cooked. I picked up a suspiciously cheap panel for my T5 build and ran it through a Victron MPPT — the charge controller's history tab is genuinely your best friend for spotting whether it's consistently underperforming over a few days rather than just blaming one grey Tuesday.

£40 is essentially a gamble worth taking if you've got a way to actually test it rather than just hoping.

@T5Project lol yeah UK solar noon is doing a lot of heavy lifting as a concept rn 😅

One thing nobody's mentioned — check the junction box on the back. Dodgy marketplace panels often have loose or corroded connections in there, and that's where a lot of the efficiency disappears. Also look for delamination around the edges, any bubbling or yellowing in the cells.

Had a similar punt on a panel for my shepherd's hut last year. Voc tested fine but the j-box was basically held together by optimism. Sorted it with some proper waterproof sealant and it's been grand since.

For £40 it's worth the gamble if you go in eyes open — just don't stick it straight onto a Victron MPPT without checking it over first.

@FX_Power was probably about to mention checking the junction box — worth finishing that thought because it's genuinely important on cheap panels.

Inspect the J-box for proper potting compound and decent MC4 connectors. Counterfeit MC4s are rife on budget panels and they're a fire risk, not just an inefficiency issue. Proper Stäubli or at least decent copies should have clear branding and click firmly.

Also check the frame for delamination at the edges — hold it up to the light and look for moisture ingress bubbling under the laminate. A panel that's been stored outside face-down on a damp driveway for six months is often visually fine until it isn't.

£40 for a genuine 200W is absolutely a win if the physical condition is solid. I've picked up similar for the static caravan and had no issues, but I checked it thoroughly first.

@BayJason the junction box is absolutely the tell — if the diodes are rattling around loose in there like a marble in a tin can, walk away faster than a man who's just spotted his ex at a boat rally.