Anyone else finding their MPPT controller underreporting in cold weather?

Noticed something odd this week — my Victron SmartSolar 100/30 has been showing lower wattage figures than I'd expect given the conditions. We've had clear skies and proper cold snaps (down to around 3°C in the mornings here in Yorkshire), and I know panels are more efficient in the cold, not less. I've got two 200W Renogy panels wired in series giving me an open-circuit voltage of around 88V when it's cold, well within the controller's limits.

The app is showing peak input rarely breaking 280W, when theoretically I should be pulling closer to 350W or more on a clear winter's day with that setup. Battery is a 200Ah lithium (BMS seems fine, accepting charge no problem), so absorption shouldn't be clamping it too early. I've double-checked my cable runs — 6mm² all the way from the roof, maybe 4 metres total, so voltage drop shouldn't be significant.

Has anyone else seen this with the SmartSolar range specifically, or is there something obvious I'm missing? Wondering whether it's a firmware thing, a temperature compensation setting I've not touched, or just me having unrealistic expectations of real-world vs. rated output. Happy to share screenshots from the VictronConnect app if that helps anyone diagnose it.

DodgyGrafter | 847 posts

@SimonPalmer Worth checking whether what you're seeing is actually underreporting or genuine lower output from a different cause. Cold temps should theoretically boost panel voltage (Voc rises noticeably below 25°C STC), but if your battery bank is already well charged, the MPPT will throttle back regardless of available solar.

What's your battery state of charge looking like when you notice the lower figures? If they're sitting at 90%+ the controller is doing exactly what it should — protecting the batteries rather than maximising harvest.

Also worth pulling the history data from VictronConnect and checking peak Voc readings — that'll tell you whether the panels themselves are performing correctly in the cold. What panels are you running and what's your battery chemistry?

VoltMax | 312 posts

@SimonPalmer cold weather is actually your panels' best mate — lower temps boost Voc, so your array might be pushing voltage higher than the MPPT's input ceiling and it's clipping itself silly without telling you properly 😅 had the exact same head-scratch moment with my van setup last January, Victron Connect app showed suspiciously flat numbers on a gorgeous frosty morning — turned out my panel strings were briefly kissing 103V on a 100V controller, oops 🤦 check your actual Voc vs rated spec adjusted for low temps, the calculation will haunt you

SparkySparky | 1,204 posts

@SimonPalmer something similar happened to me last January with my cabin setup. Panels were throwing out a gorgeous open-circuit voltage in the frost, but the actual harvest told a different story once I dug into it.

What caught me out was the battery temperature. My Fogstar Drift LiFePO4 cells were sitting at around 4°C, and the BMS was quietly throttling charge acceptance — the MPPT wasn't underreporting at all, it was just being told "slow down" from the battery side.

Worth checking your battery temp sensor readings in the VictronConnect app alongside the MPPT figures. If they're diverging from what you'd expect on a bright cold morning, that's usually where I'd start looking rather than blaming the controller itself.

The Victron kit is generally spot-on for accuracy in my experience — it's usually everything downstream causing the confusion.

HarbourKev | 2,156 posts

@SimonPalmer had exactly this puzzle with my setup last February. Turned out my Victron was reading accurately — the culprit was partial shading from frost that hadn't fully cleared the bottom edge of my panels by mid-morning. Even a thin strip of frosted glass will hammer your output disproportionately due to how cells wire in series.

Worth pulling up the VRM portal history and comparing your peak wattage against the exact time the sun hit full strength. If there's a lag of an hour or so before figures climb, frost shadow is your likely answer rather than any controller fault.

Also check your temperature compensation settings in the VictronConnect app — if that's misconfigured it can skew behaviour slightly, though probably not enough to explain a big discrepancy.

JakeWalker | 203 posts

@SimonPalmer classic case of "my solar is broken" turning out to be "my solar is perfect and I'm just bad at maths" 😄

Seriously though — have you actually checked your panel temps vs the datasheet Pmax figures? Cold boosts Voc but if

BirchRunner | 847 posts

@SimonPalmer worth checking whether the reporting is the issue rather than actual output. My SmartSolar logs in the app can lag behind real-time conditions, especially after a cold night when the Bluetooth stack seems sluggish.

What I've found at the cabin — panels genuinely over-producing on cold clear days (Voc spikes well above STC), but if your battery bank is already near full, the MPPT throttles back hard and that's what you're seeing logged. My Fogstar 280Ah lithium fills fast on a bright winter morning and then the controller just... idles.

Check your bulk/absorption/float transitions in the history tab. If it's hitting float by 10am, lower wattage figures make complete sense — it's working exactly as intended, just nothing left to push into.

HeatherGazer | 312 posts

@SimonPalmer cold actually boosts panel efficiency — lower cell temp means higher Voc. So your panels might be pushing voltage above what the MPPT expects and it's throttling slightly to protect the system. Worth checking your battery charge state too. If they're near full, the controller's deliberately pulling back. I noticed this loads in my shepherd's hut setup last winter — seemed like underperformance but the Victron app showed absorption mode was kicking in early most afternoons.