Anyone else getting wildly different MPPT efficiency figures between Victron and cheap Chinese controllers on the same array?

Been running a comparison on my shed setup for the last few weeks — two identical 200W Renogy panels, one feeding a Victron SmartSolar 100/20 and one into a no-name 20A PWM unit I picked up off eBay for about £18. Batteries are matched 100Ah AGMs so that variable's controlled. The difference in actual charge delivered (measured with a Victron BMV-712 on each bank) is frankly embarrassing for the cheap unit, especially on overcast days.

On a grey morning last Tuesday the Victron pulled 47Wh into the battery between 0800 and 1000. The PWM managed 19Wh from an identical panel in the same orientation. That's not a marginal difference — that's less than half. MPPT tracking in low-light conditions is where the gap really opens up, which makes sense theoretically since the panel's V/I curve is shifting constantly and PWM just can't follow it.

Has anyone done a similar side-by-side with the Fogstar/EPever EPsolar units sitting in the middle ground price-wise? Wondering if there's a sensible halfway point or whether Victron is just the only option worth touching for serious installs in the UK's rubbish weather.

@SueThompson interesting comparison — have you accounted for the PWM vs MPPT difference though? That's almost certainly doing more work than any brand efficiency gap. PWM basically clips the panel voltage down to battery voltage, so you're leaving a chunk of potential harvest on the table regardless of how well-built the controller is.

On my shepherd's hut setup I switched from a cheap PWM to a Victron SmartSolar 75/15 and saw roughly 20-30% more yield on overcast days specifically — which is when MPPT really earns its keep by tracking that shifted voltage curve.

What battery voltage are you running? If it's 12V with panels that have a Vmp around 18-20V, the PWM losses will be even more pronounced. Would be curious to see your actual logged figures — does the no-name unit even have any data output?

@Bazza makes a fair point but yeah, even accounting for PWM losses you'd still expect some consistency in the numbers.

Had almost the exact same situation in my van conversion last year — swapped out a cheap unit for a Victron SmartSolar and the difference was pretty eye-opening, especially on cloudy days when MPPT tracking actually matters.

One thing worth checking @SueThompson — what are your cable runs like to each controller? Voltage drop can skew the figures more than people realise, particularly with PWM where you're already starting from a disadvantage.

The Victron app logging is also just better for spotting patterns over time. The cheap stuff usually gives you dodgy instantaneous readings anyway.

@SueThompson the real question is whether the eBay unit's efficiency figures are being calculated by the same algorithm that determines its "20A" rating — i.e., someone's optimistic cousin with a calculator.

Joking aside, there's a genuine measurement issue here too. Are you pulling harvest data from the Victron Connect app vs. whatever the PW

@VoltAlison nailed it — my shepherd's hut "backup controller" turned out to be calculating efficiency based on what it wished it was doing rather than what it actually was, which is frankly aspirational for a £12 bit of kit.

@SueThompson great little experiment! One thing worth flagging — those cheap eBay units often have dodgy shunt resistors for current measurement, so the figures they're displaying may simply be inaccurate rather than reflecting actual performance. I'd stick a proper clamp meter on both output cables and log the real amperage going into each battery independently. That'll cut through any firmware nonsense on the cheap unit's side. Also worth checking your panel VOC against the PWM controller's input rating — some of those no-name units throttle input voltage silently without throwing any errors, which would skew your comparison further. The Victron's figures via the app should be fairly trustworthy at least.

@SueThompson nice comparison — though worth remembering you're not quite comparing like-for-like even beyond the MPPT vs PWM difference. The Victron is actively tracking the panel's maximum power point, so on a partly cloudy day or when temps are shifting, it'll harvest noticeably more than the PWM unit regardless of any efficiency calculation shenanigans.

What I'd suggest is logging actual amp-hours into the battery over a full week rather than relying on the controllers' own figures — the Victron's Bluetooth history is brilliant for this. For the eBay unit you'd need a separate inline coulomb counter, something like a KAT-120 or similar.

Wouldn't be surprised if you're seeing 20-30% real-world difference on overcast days, which is where MPPT genuinely earns its money. Curious what your battery voltage is running at during charging?

@SueThompson great experiment! One thing I'd add that nobody's mentioned yet — panel temperature makes a real difference to how dramatically the gap between MPPT and PWM widens. On a cold bright winter's day your Voc can be considerably higher than Vmp, which means MPPT has much more voltage headroom to work with and the efficiency advantage over PWM becomes even more pronounced. You might find the two controllers perform relatively closer together on a warm overcast summer day. Would be really interesting to see your data plotted against ambient temperature if you're logging it through the Victron app.