Anyone else running a Victron SmartSolar alongside cheap eBay panels? Curious what efficiency you're actually seeing

So I picked up four 200W "Acopower" panels off eBay back in March for about £280 the lot, which felt like a steal at the time. Running them in two series pairs into a Victron SmartSolar MPPT 100/30, feeding a pair of 100Ah leisure batteries in my Transit conversion. On paper I should be pulling close to 800W peak, but in practice I'm seeing maybe 520–560W on a good clear day, even around solar noon.

I know UK irradiance is never going to match the test conditions these panels are rated at (STC is 1000W/m² and 25°C cell temp, which basically never happens here), but I'm wondering how much of the gap is down to the panels being optimistically rated versus actual losses in the system. The Victron app is showing decent MPPT tracking and the wiring is 6mm² throughout, so I don't think I'm losing masses in cable resistance.

Has anyone done a proper back-to-back comparison with brand-name panels like Risen or Longi against the cheaper eBay stuff? I'm genuinely not sure whether splashing out on better panels would make a meaningful difference to real-world yield, or whether it's just paying for a shinier label. Would love to see some actual numbers if anyone's got them.

@TransitDream interesting setup — I'm running something similar on my static caravan, four panels of questionable Chinese origin through a SmartSolar 100/50, and honestly the MPPT controller does most of the heavy lifting in terms of squeezing efficiency out of mediocre panels.

Thing I'd check is your actual Voc under cold conditions — cheap eBay panels often have wildly optimistic specs, meaning your series string voltage could spike higher than you're expecting in January and potentially nudge uncomfortable territory on the controller input.

The Victron app will show your harvest history — worth comparing peak daily yield against theoretical maximum given your panel wattage and daylight hours. I was getting roughly 65-70% of theoretical on my dodgy panels before I re-angled the mounting, jumped to about 80% after.

Also — what's your battery chemistry? That changes the whole efficiency picture significantly.

@TransitDream the Victron doing the heavy lifting is the key factor here — the MPPT algorithm is genuinely excellent at squeezing whatever's available out of mediocre cells. I've got a similar situation on my tiny house build: four 250W panels from a supplier that shall remain nameless, paired with a SmartSolar 150/45. VRM data shows I'm consistently hitting 78–82% of the theoretical peak on clear days, which honestly surprised me.

The bigger issue with cheap panels isn't day-one efficiency — it's degradation rate. Worth logging your yield monthly through VRM so you've got a baseline. Some no-name cells drop noticeably within 18 months.

Also worth checking your string voltage at cold temps — cheap panels often have optimistic Voc specs, and on a sharp January morning you can push close to your controller's 100V ceiling unexpectedly.

Curious about the actual wiring setup you've got — are those two series pairs then wired in parallel into the controller?

I've got a similar situation on my static caravan, mixing a couple of Renogy panels with some no-name 175W units I grabbed cheap. The Victron SmartSolar handles it reasonably well but I did notice the MPPT seems to struggle a bit when the cheaper panels mismatch badly on partly cloudy days — like it's hunting for the peak more than usual.

Worth logging the data through the VictronConnect app if you haven't already. You can see exactly what's happening with the voltage and current curves. Helped me spot that one of my dodgy panels was dragging the whole string down more than I'd realised.

@TransitDream I ran a very similar experiment on my motorhome — four budget 200W panels (Renogy-branded but almost certainly same factory as half the eBay stuff) feeding a SmartSolar 100/30. What I found was the Victron's VRM logging is your best friend here; pull the yield history and compare against a solar irradiance calculator for your location. My panels were consistently hitting about 78–82% of rated output under ideal conditions, which for the price paid is perfectly acceptable.

One thing worth checking: those cheap panels often have higher-than-spec Voc in cold morning conditions. Run the Victron MPPT calculator with actual measured Voc from your panels rather than the datasheet figure, especially if you're wiring series pairs — you don't want to be nudging the 100V input limit in January.

@RussWebb the series-parallel config he's describing is solid; parallel combiner fusing at each string is essential though.

My four "300W" eBay specials peaked at 187W on the best day of July — Victron Connect doesn't lie, but apparently eBay sellers do.

@TransitDream worth checking your actual Voc and Isc against the datasheet figures before drawing conclusions about efficiency losses. I did exactly this exercise on my motorhome setup — Victron SmartSolar 100/50 with four panels that claimed 200W each. Used a clamp meter and multimeter at solar noon on a clear day, and found my Isc was about 12% below spec. Turned out the cells were genuinely undersized, not a wiring or controller issue.

The SmartSolar's MPPT algorithm is solid — I've never found it to be the bottleneck. Your losses are almost certainly in the panels themselves or your cable runs. What cross-section are you running from roof to controller? Undersized DC cabling on a motorhome is surprisingly common and can easily account for 8-10% loss before the controller even sees the current.