Anyone else noticed their MPPT readings go completely daft in this winter sun?

Been out in the van this week up in the Lakes and despite it being absolutely brass monkeys (-3°C overnight, barely 6°C by midday), my Renogy Rover 40A MPPT has been throwing up some surprisingly high voltage readings first thing in the morning. Talking 22-23V open circuit off a single 200W panel that's nominally rated at 20.5V Voc. Did a bit of reading and apparently cold temps actually increase panel voltage — something like -0.3% per °C below STC — but it still caught me off guard seeing numbers like that on the display.

The flip side is the amps are obviously dismal. I'm lucky to see 3-4A peak on a clear day this time of year, and that's maybe a 2-3 hour window before the sun drops behind the fell. Running a 100Ah LiFePO4 (Fogstar Drift) so at least I'm not stressing the battery with partial states of charge the way I used to with my old AGM.

Has anyone found a practical workaround for genuinely useful winter charging up in hilly or northern areas — whether that's panel tilt angles, adding a second panel, a small wind turbine, or just accepting the genny comes out in December? Curious what setups people are actually running rather than what looks good on paper.

@LakelandSolar yeah this is totally normal mate — cold panels actually perform better than hot ones. Something about lower cell resistance means higher Voc. Worth checking your panel spec sheet for the temp coefficient figures.

The flip side is if it got really cold overnight, your Voc could spike quite high at dawn before the controller's had chance to settle. Worth double-checking your array Voc (at that temp) doesn't exceed your MPPT's max input — caught me out once on my static van setup.

Victron's BatteryProtect app shows this really clearly if you ever fancy upgrading, logs all the weird spikes. Cold sunny days in winter are honestly peak solar efficiency days here in the UK, enjoy it!

Spot on @Midge52, and to add a bit more to that — the effect is actually more pronounced than most people expect. Your panel's Voc (open circuit voltage) rises significantly in cold temps, which is why it's absolutely crucial to check your MPPT's maximum input voltage rating before connecting a string of panels in winter. I've seen people catch out by this, especially running multiple panels in series — what's fine in July could potentially fry your controller come January if you haven't done the calculations.

@LakelandSolar worth having a look at your panel's temperature coefficient spec on the datasheet and running the numbers for your coldest expected temperatures. Renogy's own sizing tool handles this reasonably well. Enjoy the Lakes — jealous of you, it's stunning up there even in the cold! ❄️

StormyNomad | 847 posts | ⚡ Solar Enthusiast

Worth flagging something else while we're on this topic — on those really cold mornings, keep an eye on your charge controller's Voc (open circuit voltage) rating. Cold panels can push voltage noticeably higher than the manufacturer's STC figures, and if you're running panels close to your MPPT's maximum input voltage you could theoretically exceed it. Caught me out years ago before I understood what was happening.

As @TerryBurns says, the effect is more pronounced than people expect — I've seen my panels sitting in the Lakes in January throwing 10-15% above their rated Voc. Not a problem if you've got headroom built in, but worth double-checking your specs before adding another panel to your array over winter. Stay warm up there @LakelandSolar! 🥶

What @StormyNomad is alluding to is worth expanding on — Voc at low temperatures can genuinely exceed your MPPT's maximum input voltage, which is where things get expensive.

On my tiny house setup I'm running four 200W panels and I had to derate the string specifically to account for winter Voc. The calculation is straightforward: take your panel's temperature coefficient for Voc (usually around -0.3%/°C), work out how far below 25°C your coldest expected temperature is, and apply accordingly.

At -3°C you're 28°C below STC, so roughly +8.4% on your rated Voc per panel. String multiple panels and that headroom disappears fast.

Victron's MPPT range handles this gracefully — their wider input voltage windows give you genuine safety margin. The Renogy Rover is decent but check the absolute maximum input spec in the datasheet before congratulating yourself on the good readings.

Just to add a practical angle here — it's worth keeping an eye on your battery temperature as well as the panels. A cold battery has higher internal resistance, so even if your MPPT is doing its job beautifully and tracking those elevated cold-weather voltages, the charge acceptance can still be poor first thing in the morning. I've found with my system that the first hour or so after a cold night the readings can look a bit odd precisely because the battery isn't accepting charge efficiently yet. Give it time to warm up a bit and things usually settle down nicely. @LakelandSolar — how are your batteries situated in the van? If they're in an uninsulated bay they'll be stone cold too. Wrapping them in some decent insulation makes a surprising difference in winter.

SolarPaul | 1,203 posts | 🔆 Off-Grid Addict

Great point from @MistyMaker about battery temp — worth adding that if you're running lithium (LiFePO4 particularly), most BMS units will flat-out refuse to accept charge below around 0°C, so even if your MPPT is producing cracking numbers, that energy's going nowhere useful. You might see your controller looking busy whilst the batteries sit there doing absolutely nothing.

If you're in the Lakes overnight @LakelandSolar, worth checking whether your BMS has low-temp charging cutoff and if so, whether it's actually triggering. Some cheaper units aren't always reliable about it either way — both false triggering AND failing to protect can be an issue.

A cheap battery temperature sensor wired to your MPPT (if it supports one) sorts a lot of this out automatically.

Rachel1995 | 347 posts | 🔴 Van Life Convert

Really interesting thread this! Just to add something nobody's mentioned yet — the combination of cold air and snow or frost on the ground can actually boost your readings further through reflected light (albedo effect). I noticed this last February in Scotland, my panels were genuinely outperforming their rated specs on a crisp bright morning with snow all around. Caught me off guard the first time! Worth factoring in if you're parked somewhere with good ground reflection. @LakelandSolar the Lakes can definitely throw that at you! 🏔️

Scouse | 2,847 posts | ⚡ Narrowboat & Caravan Obsessive

Worth noting the actual physics here — cold panels have lower internal resistance, which genuinely boosts Voc significantly above the STC rating (measured at 25°C). On my static caravan setup with a Victron SmartSolar 100/30, I've logged open-circuit voltages nearly 15% above the panel's rated spec on crisp winter mornings. That's not a fault, that's thermodynamics working in your favour.

@LakelandSolar — just make sure your MPPT's maximum input voltage rating comfortably exceeds what you're seeing. If you're stringing multiple panels, winter Voc can catch people out badly.

GlenDixon | 892 posts | ⚡ Off-Grid Enthusiast

Great thread @LakelandSolar — the Lakes in winter is absolutely stunning but brutal for kit! One thing worth mentioning that nobody's touched on yet: that cold, clear air often comes with excellent atmospheric transparency, meaning less diffuse scatter and more direct irradiance hitting your panels. Combined with the lower cell temperature improving efficiency, you can genuinely exceed your STC-rated wattage temporarily. I've seen my own setup nudge past its nominal figures on crisp February mornings in Scotland. Don't let it alarm you — it's actually your system performing better than the spec sheet suggests it should!

FormerMechanic15 | 1,203 posts | 🔪 Shepherd's Hut Dweller

Same thing happens on my hut setup. Got a Victron SmartSolar 100/30 and after a hard frost the morning voltage spike can trip the over-voltage warning if I'm not careful — panels read well above their rated Voc sometimes.

Worth keeping an eye on your battery bank temp too. Cold LiFePO4 won't accept charge properly below about 5°C — Fogstar cells I'm running have a built-in low-temp cutoff which has saved me a few times.

@Scouse is right about the physics. Just make sure your MPPT's voltage ceiling can handle what the panels throw at it on a crisp clear morning.