Anyone else noticed their MPPT efficiency drop noticeably in winter vs summer?

Running a Victron SmartSolar 100/30 on the Ducato with 400W of Renogy panels (2x200W in series). Summer I was regularly seeing 85–90% efficiency pulling decent amps into a 200Ah Fogstar Drift lithium. Now in January, even on clear days, I'm lucky to hit 70–75% and the controller feels like it's hunting around the MPP more than usual.

I've checked the obvious stuff — connections are clean, wiring's the same gauge (6mm² from roof to controller), no shading issues. My suspicion is the Voc is climbing because of the cold temps (panels hitting -2°C overnight, still cold at 9am when generation starts), and the MPPT is struggling to track efficiently at those higher input voltages. At 2°C panel temp the Voc on these 200W panels sits around 50–51V per panel, so series that's 100–102V — right near the controller's 100V absolute max. Wondering if it's actually clipping or just being cautious.

Has anyone data-logged their SmartSolar through a proper cold snap and seen similar behaviour? I can pull graphs off VictronConnect but I'm not sure what I'm looking for exactly to confirm clipping vs poor tracking. Would be interested to hear whether anyone's resolved this by reconfiguring panel layout (parallel instead of series) to keep Voc lower, even if it means higher current and thicker cable runs.

@DucatoSolar yes! Really curious about this actually — I've noticed similar on my tiny house setup with a Victron 100/20.

Quick question though: are you factoring in the panel voltage differences? In winter my Voc shoots up noticeably in the cold, which I think affects how the MPPT algorithm tracks?

Also wondering whether your panels are getting any shading from lower sun angles that wasn't happening in summer — even partial shading on a series string can hammer efficiency quite dramatically can't it?

What does your VRM data show for the tracking efficiency specifically? Mine's gone a bit erratic and I can't tell if it's genuinely the MPPT working harder or just the panels underperforming. Would love to compare actual numbers if you've got them logged somewhere 🙂

@DucatoSolar worth separating out a few things here — lower sun angle means your panels aren't hitting rated wattage in the first place, so the MPPT has less to work with. That's not really an efficiency drop, that's just physics.

What I'd actually watch is your panel voltage in cold mornings — lithium acceptance voltage and cold Voc can sometimes cause the controller to throttle more than you'd expect.

Running a similar Victron setup on my garden office with Fogstar cells and honestly the MPPT efficiency percentage itself stays pretty solid year-round. Total yield is obviously way down, but that's the panels, not the controller.

Worth logging a few days in VictronConnect and checking the actual efficiency figure rather than going by feel — might surprise you.

Hey @DucatoSolar — one thing worth adding that often gets overlooked: cold panels actually perform better electrically (lower cell temperature = higher Voc), so your MPPT might briefly look more efficient on a crisp clear day. The drop you're likely seeing is more about the battery side — lithium acceptance rates fall off in the cold, so the Fogstar may be throttling charge current before the MPPT can do its thing properly. Check your Victron app and see if you're hitting absorption voltage quicker than expected. Also worth checking whether your wiring connections have any resistance that only becomes apparent when you're pushing lower currents over longer periods. @CotswoldExplorer makes a fair point about separating variables — logging a few days on VRM would give you something concrete to work with rather than guessing.

@Jonno71 that's actually really useful to know — so the panel output side isn't necessarily the problem in winter, it's more about what the MPPT is doing with lower, inconsistent power?

I'm seeing this across a couple of my setups — the boat and the shepherd's hut both running Victron SmartSolars — and the hut especially struggles on overcast days. I'd assumed it was purely irradiance but wondered if the MPPT algorithm itself takes longer to "hunt" for the true MPP when input wattage is low and unstable?

Does anyone know if the Victron firmware handles this differently compared to cheaper controllers? I've got a basic Renogy unit on the backup system and curious whether it's actually worse at tracking in marginal conditions or whether I just assume it is because it's cheaper.

@Jonno71 makes a fair point on cold panels — saw my Voc jump noticeably on a crisp clear morning last week, actually had to double-check it wasn't a fault 😄

From my shepherd's hut setup I'd add: shading geometry in winter is brutal. Trees and hedgerows that were fine in July are now throwing shadows across my panels from about 2pm. Even partial shading on a series string tanks the whole thing.

Worth logging properly with the Victron app — the MPPT efficiency numbers on their own don't tell you much without knowing actual irradiance. Could be your panels are just receiving less usable light rather than the controller losing efficiency.

@TracyKnight that Voc jump is why you want to double-check your series string doesn't nudge past the MPPT's input ceiling on a cold morning — 100V limit on the 100/30 gets closer than people think when panels are at -10°C and you're doing the maths properly with temperature coefficients rather than just the STC figure on the spec sheet.

My efficiency dip in winter is almost entirely down to the battery sitting at lower SoC more often (less sun = less charge = more time in bulk = lower apparent efficiency if you're measuring crudely), rather than anything the MPPT itself is doing wrong.

Really good point from @T5Project on the Voc ceiling — worth anyone in a similar setup actually doing the cold Voc calculation rather than just assuming you're fine because summer figures looked okay. Beyond that, I'd add that winter efficiency dips on the MPPT itself are often partly down to reduced panel temperature differential from ambient — your controller's conversion losses become a bigger proportion of a smaller harvest. Not much you can do about that, but it's worth distinguishing between genuine controller inefficiency and just the maths working against you on low-irradiance days.

Good shout from @OakTel and @T5Project on the Voc calculations — worth doing properly rather than guessing. On the efficiency drop question though, I'd also look at your battery state more carefully in winter. Lithium BMS often starts throttling charge acceptance when cell temps drop below about 5°C, so your MPPT might actually be working fine but the battery is refusing the current. Worth checking your Fogstar's temperature specs. I've seen people blame the controller when really it's the cells being cautious, which is fair enough honestly — better that than damage.

Good points all round on the Voc side. Slightly different angle — has anyone looked at whether the drop in efficiency correlates with the battery's state of charge behaviour in winter? I'm wondering if my setup is spending more time in absorption/float phases because the lithium isn't getting properly cycled day-to-day, which would naturally drag the apparent MPPT efficiency figures down compared to summer when you're pulling the bank low and charging hard from a proper deficit. Curious whether @DucatoSolar is comparing like-for-like SOC conditions or just raw seasonal numbers?

Good points all round on the Voc stuff. On the efficiency question though — worth remembering that MPPT controllers generally perform better with a wider voltage differential between panel input and battery. In winter your panels are often running closer to battery voltage anyway due to lower irradiance, so the controller has less to work with regardless of temperature. Cold temps help Voc but if you're losing hours of usable generation, the overall harvest just drops. Have you checked your actual panel voltage under load in winter @DucatoSolar? Sometimes the numbers tell a different story than the efficiency percentage alone.

Really interesting thread this. One thing I'd add to what @Paddy78 and @MoorSeeker have touched on — don't overlook the effect of reduced panel temperature on your actual power output figures. Cold panels theoretically produce more voltage, which sounds great, but if your real-world wattage is still lower due to minimal sun hours and low irradiance, your controller's working harder for less reward. I've found logging via the VictronConnect app over a few weeks really helps spot patterns rather than just eyeballing it. Makes it easier to separate genuine efficiency losses from just... winter being rubbish.

Great thread. To add something nobody's mentioned yet — have you checked your battery temperature? The Fogstar Drift's BMS will reduce charge acceptance in cold weather, which means the MPPT can't always deliver what it's generating. So even if the controller itself is running efficiently, the battery is essentially throttling the input. If you're not already using a Victron battery temperature sensor, it's worth adding one — it lets the SmartSolar adjust charge voltages accordingly and you'll likely recover some of that apparent efficiency loss. Makes a noticeable difference once temperatures regularly drop below 10°C.

Pedant hat on for a sec 🎩

Worth clarifying what "efficiency" actually means here — are you measuring MPPT conversion efficiency (typically 97–98% regardless of season on a decent Victron) or harvest efficiency relative to panel rated output? Because they're very different things.

If your controller is genuinely dropping below ~94% conversion

Good shout from @DucatoCamper on the battery temp side. One thing nobody's picked up yet — low sun angle in winter means your panels are rarely hitting anything close to rated output even on a clear day, so the MPPT's got less headroom to work with. I've noticed on my garden office setup (400W fixed tilt at 35°) that December efficiency looks worse partly because the controller's just not getting the juice to optimise properly. Try logging your PV input watts vs output — might find the controller itself is fine, it's the source that's the bottleneck.