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 about 2°C in the mornings), and my two 200W panels should theoretically be putting out more in the cold, not less. VOC is reading around 78V which seems about right, but the actual charge watts are hovering around 180-200W when the sun's out rather than the 300-350W I'd hope for.

Battery bank is a 200Ah lithium (Fogstar Drift), so I don't think it's being throttled by charge acceptance — it was sitting at around 40% SOC when I noticed this. The Victron app is showing everything green and no fault codes, which is making me scratch my head even more.

Has anyone come across this before? Could it be a wiring issue on the PV side, or is there something in the MPPT settings I should be looking at? I've got 6mm² cable running about 4 metres from the panels to the controller, which I didn't think would cause much of a drop.

@RobRobinson78 actually wondering if it might be the opposite problem — cold panels can exceed their rated Voc, which sometimes pushes voltage above what the MPPT can handle and it clips or shuts down briefly. Worth checking your array's cold-weather Voc calculation against the controller's max input voltage.

On my shepherd's hut setup I noticed similar figures last January. Turned out my Fogstar lithium bank was already near full by mid-morning, so the Victron was throttling back naturally. Are you watching the wattage when the batteries are still genuinely depleted, or partway through the day when absorption's already kicking in?

What's your battery SOC looking like when you're seeing the low figures?

@RobRobinson78 @SOC_Wizard is onto something crucial here — this isn't underreporting, it's almost certainly voltage clipping.

Cold panels push Voc well above the STC rating (roughly -0.3%/°C on most crystalline cells). At 2°C you could easily see 10-15% higher Voc than the datasheet suggests. If that's nudging your string voltage toward or beyond the SmartSolar's 100V input limit, the MPPT backs off to protect itself — which looks like underperformance.

Check your VictronConnect history graph — if input voltage is suspiciously close to 100V on those cold mornings, that's your culprit.

Quick fix: review your string configuration. You may need to drop a panel from the series string, or consider upgrading to a 150V unit. Had exactly this scenario on my cabin array last January — cost me about 20% harvest until I reconfigured.

Great points from @SOC_Wizard and @DailySolar already. One thing worth checking on your SmartSolar is the battery temperature compensation settings — if your battery bank is sitting in an unheated space, the controller may actually be pulling back charge current to protect the cells, which would show as lower wattage figures even when solar input looks promising.

Also worth connecting via the VictronConnect app and checking the "bulk/absorption/float" status history. If it's hitting absorption voltage earlier than expected due to a cold, partially-discharged battery behaving oddly, it'll throttle back production figures.

What battery chemistry are you running? AGM and gel behave quite differently to lithium in these temperatures, and the compensation curve makes a real difference to what the MPPT actually does versus what the panels are theoretically capable of producing.

Had this exact drama with my motorhome setup last January — panels were pushing voltage so high the Victron was basically throttling itself to protect the batteries, which looked like underperformance but was actually the controller doing its job properly. Check your VictronConnect app history and see if input voltage was spiking near your array's max — dead giveaway.