Anyone else running a budget 12V system off cheap eBay panels? What are your actual real-world numbers?

Picked up two 100W mono panels off eBay for about £38 each last spring — some no-name brand, can't even find the seller anymore. Stuck them on the shed roof with a basic tilt bracket I bodged together from some 40mm aluminium angle from Screwfix. Running into a Victron 75/15 MPPT (the one decent bit of kit I splashed out on) and a pair of 100Ah leisure batteries wired in parallel.

Honest numbers: on a decent sunny day in July I was seeing peaks around 160W actual output, which felt reasonable given the losses. But now we're into the grey months I'm lucky to pull 30–40W most days, and the batteries are sitting at around 70% most mornings. I know that's just winter in the UK being winter, but I'm wondering if I'm losing more than I should somewhere — maybe the panels themselves are genuinely poor quality, or my wiring runs are too long (about 6 metres from panels to controller, using 4mm² cable).

Has anyone done a proper before/after comparison swapping cheap panels for decent Renogy or similar? Or is the Victron doing enough to compensate for dodgy panels that it's not really worth upgrading? Tempted to add a third panel to brute-force the winter problem but not sure if that's just throwing money at the wrong issue.

@Chippy33 Right, so here's the thing nobody tells you about those panels — the "100W" rating is under STC conditions that you'll basically never see in the UK. I've got a pair of similar specced panels on my static caravan in [LOCATION REDACTED] and on a decent summer day I'm pulling maybe 65-70W peak per panel, realistically. Overcast days? Don't make me laugh. 20-30W if you're lucky.

The actual problem with no-name eBay panels isn't necessarily the cells — it's the junction box quality and whether those bypass diodes are actually functional. Cheap encapsulant delamination is also a genuine concern after a couple of UK winters.

What MPPT controller are you running? Because if you've got some generic PWM controller paired with those panels you're leaving even more power on the table.

@Chippy33 Sounds familiar! I've got three of those mystery eBay panels running a similar setup. Honest numbers from last summer: on a decent July day I was seeing about 85-90W peak from each "100W" panel, which I thought was reasonable given the price. Winter's a different story though — December and January I was lucky to average 20-30W across the day.

One thing worth checking is your actual terminal voltage under load. Mine were advertising Vmp of 18V but I was consistently measuring 16.8V on warm days, which does affect your overall yield.

What charge controller are you using? I switched from a PWM to an EPEVER MPPT last year and gained a noticeable improvement even with the dodgy panels. Worth the investment if you haven't already made that jump.

Really useful thread this. I've got four of the same sort of panels — paid similar money, no idea who made them.

One thing worth mentioning that nobody's touched on yet: I've noticed significant variation between panels from the same batch. Mine tested anywhere from about 68W to 89W actual output on a clear day. So if you're wiring in series, your weakest panel drags everything down. Worth testing each one individually before committing to your configuration.

Also check the junction boxes on those cheap ones — two of mine had pretty poor waterproofing from the factory and I had to reseal them myself with some silicone. Nothing catastrophic but worth a look before winter sets in properly.

@Chippy33 what charge controller are you running with them? That makes a massive difference to what you actually pull from budget panels in my experience.

Really useful to hear everyone's real-world numbers. I've had similar panels running for about 18 months now and one thing I'd add — keep an eye on how they perform after a good rain versus a dry spell. Mine drop noticeably when dust and bird muck builds up, we're talking maybe 15-20% in my experience. A quick wipe with soapy water made a genuine difference. Also worth checking your connections periodically; I found one of my MC4 connectors had developed a bit of corrosion which was quietly robbing performance for weeks before I spotted it. Nothing dramatic, just slightly green around the join. @RussWilson the voltage drop you're seeing might be worth investigating there first before assuming it's the panel itself.

Really interested in this thread — I'm about to do something similar for my garden office build and trying to decide whether to go cheap eBay panels or splash out on something like Renogy.

One thing I'm wondering though: how are you all wiring them? Series or parallel into your charge controller? I've been reading conflicting stuff about whether cheap panels handle series connections reliably when the specs are slightly off between panels.

Also @Chippy33 — what charge controller are you running with them? I've got a Victron SmartSolar sitting in a box waiting to go, and I'm slightly nervous about pairing decent kit with mystery panels. Anyone else running a quality MPPT with budget panels, or does it feel like overkill?