Anyone else finding their MPPT controller underreporting on cloudy days?

Noticed this with my Victron SmartSolar 100/30 last winter — on heavy overcast days the reported wattage seemed suspiciously low compared to what my clamp meter was showing going into the batteries.差distance wasn't massive, maybe 8-12%, but it was consistent enough to make me suspicious.

My current thinking is it's less about the MPPT underreporting and more about how it handles the weird voltage/current behaviour you get from panels in diffuse light. The I-V curve changes significantly when you're pulling from a low irradiance sky — the controller's tracking algorithm can get a bit lost and you end up not extracting the theoretical maximum anyway. So the "loss" might be partly real inefficiency, not just a display error.

That said, I've also seen threads suggesting the shunt-based measurement in some cheaper units (not Victron, to be fair) drifts at lower current levels where accuracy matters less to the designers.

A few questions worth asking:

• Are you comparing against a calibrated reference, or just gut feel?
• What panels? Some cheap Renogy-branded stuff I've seen has optimistic STC ratings to begin with
• Have you checked your wiring resistance? Voltage drop across a dodgy connection would show up exactly like this

I run a small setup on my shepherd's hut — 2x 200W panels — so I'm not dealing with huge numbers, but even small percentage losses matter when you're trying to squeeze through a grey November week.

Curious whether anyone's done a proper side-by-side with a calibrated inline meter to actually quantify this rather than relying on the app readouts.

BazKnight57 | Posts: 847 | Location: North Yorkshire

@ExPostie interesting one this. I've noticed something similar with my EPEver Tracer but I'd actually suspect the clamp meter before the MPPT in most cases — those cheap clamp meters really struggle with accuracy at low currents, which is exactly what you're dealing with on a murky winter day. Worth checking what your clamp meter's minimum rated current is.

That said, Victron's voltage and current sensing isn't perfect either, and temperature compensation can sometimes skew the reported figures slightly when it's cold.

Have you tried cross-referencing against the SmartSolar app over a longer period rather than spot-checking? The daily yield totals tend to be more reliable than instantaneous readings. Also worth checking your shunt calibration if you're running one.

MrBodge65 | Posts: 312 | Location: Array

Had exactly this on my narrowboat setup last winter. The Victron SmartSolar reports input wattage calculated from panel voltage × current, but on heavy overcast the panels drop well below their rated voltage — sometimes into a weird zone where the MPPT algorithm is still hunting for the true MPP.

Worth checking: is your clamp meter reading after the controller (battery side)? Because conversion losses mean the two figures genuinely won't match anyway — controller efficiency drops noticeably at low input power.

I logged mine through VRM portal and cross-referenced with a separate Victron BMV-712 on the battery. The discrepancy was mostly real, not a reporting bug — the MPPT just isn't operating efficiently when panels are putting out 15-20W in December cloud.

Shading on even one cell will absolutely tank the whole string too. Worth ruling that out first.

BayLisa | Posts: 203 | Location: Array

Had this on my motorhome setup — drove me mental for weeks. Turned out my panel voltage was dropping so low on proper grey days that the MPPT was bouncing in and out of tracking. Victron app showed these tiny little dips if you zoomed into the graph.

Worth checking your battery voltage at the time too. If it's nearly full, the controller throttles back anyway and the numbers look weird. Easy to mistake that for underreporting when it's actually just doing its job 🙄

Also — are your panels dirty? Sounds obvious but a murky panel on an already rubbish overcast day is basically producing nowt, and the figures will match that accurately even if it seems wrong.

@MrBodge65 narrowboat same issue? Condensation on panels maybe?

DaleBen | Posts: 1,204 | Location: Shropshire

Worth checking your battery voltage during those cloudy periods too — if the bank is already fairly well charged, the MPPT will throttle back to avoid overcharging, so what the controller reports is genuinely lower, not a measurement error. Your clamp meter's catching the instantaneous current before any smoothing algorithm kicks in, whereas Victron averages things out over a short window.

Try deliberately running a decent load whilst it's overcast and see if the reported figures climb. If they do, it's absorption/float throttling rather than a dodgy reading. @ExPostie this is especially likely if you've got a smaller battery bank relative to your panel capacity. The 100/30 is a solid unit — in my experience they're pretty accurate, so I'd suspect the charging state before blaming the controller itself.

HollyGaz | Posts: 87 | Location: Array

Ooh interesting one — I've got a SmartSolar 75/15 on the van and a 100/20 on the narrowboat so this is relevant to me on two fronts!

Quick question for the thread: are you lot seeing this only on heavily overcast days, or does it happen on that classic British "bright but hazy" sky too? Because I swear mine goes a bit odd on those bright-grey days where you'd expect decent output.

Also — has anyone cross-checked using the VictronConnect app's historical data against an external logger? I've been meaning to pick up a Renogy battery monitor to run alongside the Victron just to have a second opinion, but haven't got round to it yet.

@DaleBen makes a good point about battery voltage — if you're already near absorption voltage the controller will naturally throttle back, which could explain some of it?