Anyone else finding their MPPT controller readings all over the place this time of year?

Been scratching my head at my Victron SmartSolar 100/30 for the past few weeks. We're in mid-January, panels are 2x 200W mono on the van roof, and I'm getting some really inconsistent numbers — yesterday hit a decent 18A around midday with clear skies, but this morning under thin cloud I barely saw 3A even though the voltage looked fine sitting around 18V open circuit.

I know low sun angle is part of it, obviously, but what's throwing me is how dramatically it swings even on days that look similar weather-wise. I've had a poke around the VRM portal and the yield graphs look like someone's drawn them with their non-dominant hand. My 200Ah lithium (Fogstar Drift) is sitting healthy enough, so it's not a load or battery issue as far as I can tell.

Has anyone else noticed this with their setup over winter, particularly if you're parked up in the north of England or Scotland? Wondering whether it's just the reality of January solar or if there's something worth investigating on the wiring or settings side of things.

@StormyWarden January is just brutal for MPPT readings, full stop. Low sun angle means your panels are barely skimming the horizon for most of the day — you'll get a decent spike around solar noon then practically nothing either side of it.

Two things worth checking on the SmartSolar app:

1. Absorption voltage — cold panels can push Voc higher than your controller expects, so make sure your temp compensation is set correctly
2. Shading — even a thin shadow across one cell at this sun angle absolutely hammers output on mono panels

My garden office setup sees maybe 30–40% of rated output on a clear January day compared to June. That's not a fault, that's just physics.

What does your yield history graph actually look like? If it's consistent day-to-day it's probably fine. Random drops and spikes are a different problem.

Yeah, what @GoldenSocket said, but I'd also add — have you checked whether you're getting partial shading issues? This time of year the sun's so low that things which cast zero shadow in summer (roof bars, a nearby tree, even a neighbouring vehicle if you're parked up) can suddenly clip the corner of a panel for hours at a time. With two panels in series that'll absolutely hammer your MPPT tracking. Try logging your readings against your exact parking orientation for a few days. I did this last winter and realised my nearside panel was being shadowed by a rooftop vent from about 10am to 1pm — basically wiping out my peak window entirely. Also worth checking your battery temperature if you haven't, as cold batteries change the absorption voltage target and can make the controller look like it's behaving oddly.

Hey @StormyWarden, one thing nobody's mentioned yet — have you looked at your panel voltage during those inconsistent readings? In January the cold temps can actually push your Voc noticeably higher than the rated spec, which is fine, but if your MPPT is spending ages hunting for the true peak power point on a partly cloudy day (when irradiance is jumping around constantly), you'll see the wattage figures bouncing all over the shop. Totally normal behaviour really. Also worth checking your battery state of charge — if you're regularly hitting absorption by late morning with depleted winter sun, the controller will throttle back and the numbers look weird compared to what you'd expect. The Victron app history graphs are your best friend here, have a proper look at the daily yield trends rather than the live figures. 👍

Really relate to this — I've got a Victron SmartSolar 75/15 on my shepherd's hut and the readings in January are genuinely baffling sometimes. One thing I'd add that nobody's touched on yet: temperature compensation. Cold panels actually produce higher VOC than your spec sheet shows, which can throw the MPPT algorithm off briefly until it settles.

Worth checking your SmartSolar app to see if temperature compensation is enabled and set correctly for your panel type. Mine was completely wrong when I first set it up.

Also curious — are these inconsistencies happening more in the morning when panels are still cold, or throughout the day? On my boat setup I noticed morning readings were the wildest until I sorted the temp coefficient settings.

@StormyWarden something worth adding that I haven't seen mentioned yet — low winter sun angle in the UK means your panels are operating at a much steeper angle of incidence, which massively affects how the MPPT algorithm finds the power curve. On my setup (3x 200W on the cabin roof), I noticed the SmartSolar sometimes latches onto a false MPP during those brief bright spells between cloud cover — it's sweeping the curve too slowly to track properly when irradiance is changing rapidly.

Worth enabling the "Equalize" sweep or just checking your MPPT algorithm settings in VictronConnect. Also pull up the history tab — if your "max power today" timestamps cluster around midday but actual Wh harvested looks low, that confirms the tracking issue rather than a panel or wiring fault.

Hey @StormyWarden! Just to add to what's already been said — one thing worth checking is whether your battery temperature is affecting the charge profile. Victron's absorption and float voltages shift with temperature, so on genuinely cold mornings (we've had some brutal ones lately!) your controller may be pushing slightly higher voltages than you'd expect, which can look odd in the app history. If you've got the Smart Battery Sense or a temperature sensor connected, double-check it's reading accurately and positioned away from any heat sources. Also worth having a look at your VRM portal history if you're logging data — the graphs make patterns much easier to spot than just eyeballing the controller display. Could save you a lot of head-scratching! 😄