Anyone else finding their PWM controller struggles in the winter months?

Been running a 200W panel (a Renogy 12V mono) into a 100Ah AGM battery via a basic 20A PWM controller since last spring and it was brilliant all summer. Now we're into January though I'm really noticing the difference. Yesterday I got barely 18Ah back into the battery on what felt like a reasonably bright day — no direct shade, panel angled up steeper than usual to compensate for the low sun.

I know PWM wastes some of the panel's potential compared to MPPT, something about not being able to harvest the extra voltage the panel produces in cold weather. My panel was sitting at around 20V open circuit this morning and the battery was at 12.4V, so there's a fair bit being left on the table I reckon. Wondering whether upgrading to even a cheap MPPT (looking at the Victron 75/15 or maybe one of the EPever units) would make a meaningful difference in these short, low-angle days.

Has anyone actually done a like-for-like comparison going from PWM to MPPT on a similar-sized system over winter? Would love to know if you saw a noticeable real-world gain or whether I'm just expecting too much from 200W in a British January regardless of the controller.

Nessa55 | 312 posts

@Chris1977 Yes, totally familiar with this! The thing people don't often realise is that PWM controllers essentially drag your panel voltage down to match the battery, so in winter when your panels are already underperforming due to low sun angles and shorter days, you're losing even more potential harvest.

The cold temperatures actually help panel efficiency slightly, but PWM just can't capitalise on that the way an MPPT controller would. MPPT finds the panel's sweet spot voltage regardless of battery state.

Worth checking your panel angle too - tilting steeper in winter makes a noticeable difference for us here in the UK. I went from about 15° to nearly 60° on mine between seasons and it genuinely helped.

An MPPT upgrade would be my longer term recommendation though, especially if you're relying on this setup through the darker months.

LochLinda | 847 posts

@Chris1977 Worth adding to what @Nessa55 is getting at — the real killer in winter isn't just low irradiance, it's panel voltage behaviour. A 12V nominal panel in cold conditions can push Voc well above 22V, and a PWM controller simply clamps that down to battery voltage, wasting all that extra potential. An MPPT controller — even a modest Victron 75/15 — would harvest that higher voltage properly and convert it to usable charge current. I made this exact switch on my static caravan setup two winters ago and saw roughly 20–25% more yield on clear cold days compared to the PWM unit I'd been running. Your 200W panel deserves better than a PWM in Scottish-style January conditions, frankly. Budget around £60–80 for a decent 15–20A MPPT and you'll recoup that in avoided battery replacement costs alone.

BatteryTim | 1,204 posts

Upgrade to MPPT. Full stop.

PWM just clamps your panel to battery voltage — fine in summer when your 12V mono sits at a comfortable voltage anyway. Winter changes everything. Panel Voc drops in the cold (actually works in your favour there) but you lose all that extra voltage headroom as heat rather than amps.

A decent 20A MPPT — Victron SmartSolar 75/15 is the obvious choice, about £70-80 — will squeeze significantly more out of a 200W panel on a grey January afternoon than any PWM ever will.

Ran PWM on my shepherd's hut setup years ago. Switched to Victron MPPT and the difference in winter charge cycles was immediately obvious. No contest.

Your 100Ah AGM is already taking a hammering in the cold too, so every amp matters.

Ray1981 | 89 posts

Good thread this. One thing nobody's mentioned yet — have you checked your panel angle? In January the sun sits really low in the sky, so if your panel is mounted flat or at a shallow pitch you're losing a significant chunk of potential harvest before your controller even comes into it. Tilting steeper in winter makes a noticeable difference. I knocked up a simple adjustable bracket last November and genuinely picked up an extra hour of decent charging on clear days.

That said, @BatteryTim isn't wrong about MPPT being the proper long-term answer, especially with a mono panel like yours which has a higher Vmp. But sorting your panel angle costs nothing and is worth doing regardless of which controller you're running.

BatteryJason | 312 posts

@Ray1981 raises a fair point on angle, but even with my shepherd's hut panel tilted like it's trying to sunbathe on a Spanish holiday, a PWM in January is essentially just a very expensive paperweight with an LED on it. 🔦

YorkshireExplorer | 312 posts

Good shout from @Ray1981 on the panel angle — that's often the quick win people overlook. I tilted mine up to around 60° for winter and genuinely noticed a difference within days.

@BatteryTim is right that MPPT is the proper solution long-term, but if budget's tight right now, also worth checking your connections and cables for any resistance issues. Cold weather can make dodgy crimps worse and you'll lose precious watts you can't afford to lose in January.

Also @Chris1977 — what's your battery temperature like? AGMs take a charge poorly when they're cold, so even if your controller is doing its job the battery itself might be refusing to accept much. If it's in an uninsulated space, wrapping it in some old blanket or moving it somewhere slightly warmer can make a surprising difference.

NeilSmith | 156 posts

Worth adding something nobody's touched on yet — at this time of year your battery itself is a significant part of the problem. AGMs lose a fair chunk of their effective capacity in cold temperatures, so even when your panel is producing decent output, you're essentially trying to fill a smaller bucket. If your battery's sitting in an uninsulated space, wrapping it in some closed-cell foam makes a genuine difference. Also @Chris1977, have you checked what voltage your PWM is actually cutting off charging? Some cheaper units have fixed charge profiles that aren't well suited to cold-temperature AGM behaviour.

Thommo75 | 847 posts

Something worth checking that nobody's mentioned yet — have you actually measured your panel's open circuit voltage on a cold clear morning? You might be surprised. Cold temperatures push Voc up noticeably on mono panels, sometimes enough that a PWM controller starts looking a bit marginal for your setup. PWM just clamps that voltage down and wastes it, whereas an MPPT would harvest the extra. Not saying go out and buy one immediately, but if you're already on the fence about upgrading, winter's actually the best time to see what you're leaving on the table. A cheap multimeter check costs nothing. 🙂

AlanPalmer57 | 43 posts

Had almost identical setup last winter and the PWM limitation really hit me hard around December/January. The fundamental issue is that PWM can't exploit the higher Voc you get from cold panels — it just clips that extra voltage rather than converting it like an MPPT would. I eventually swapped to a Victron SmartSolar 75/15 and the difference in low-light harvesting was genuinely noticeable. Obviously that's an extra outlay but worth considering if you're relying on this system year-round. What's your typical daily consumption at the moment? That might help work out whether upgrading the controller is actually worth it for your situation.