Anyone else had issues with PWM controllers undercharging in UK winter conditions?

So I've been running a 200W panel on my narrowboat since last March, paired with a cheap 20A PWM controller I picked up off eBay for about £18. Over summer it was brilliant — batteries were consistently hitting 14.4V and I had more than enough power for lighting, the 12V pump, and charging my laptop. Dead chuffed with it.

Now we're into November and I'm really struggling. Even on what I'd call a "decent" winter day here in the Midlands, I'm barely seeing 12.8V going into my 100Ah leisure battery by mid-afternoon. The panel voltage open-circuit is reading around 21V on those days, so it's not the panel itself. I've started wondering whether the PWM controller just can't cope when panel temps drop and the Vmp shifts up above what it can handle properly — from what I've read, an MPPT would harvest significantly more current in these conditions.

Has anyone actually done a proper like-for-like comparison between PWM and MPPT on a similar-sized system during a UK winter? I've been looking at the Victron 75/15 MPPT which seems to come up a lot on here, sitting around £65–£70 new. Trying to work out if that's genuinely worth the upgrade or if my issue is actually something else entirely, like sulfation starting on the battery after 8 months of use.

Reply by ExJoiner6:

@Rachel1995 This is a really common one on boats, especially once you get past the equinox. The core issue with PWM in winter isn't just the low sun angle — it's that your panel's Vmp drops closer to battery voltage when it's cold, so the controller has almost nothing useful to work with during those short grey days.

Honestly, for a narrowboat setup I'd seriously consider swapping to even a basic MPPT. Something like the Victron 75/15 can be had for around £60-70 and you'll typically see 20-30% better harvest in these conditions. It's not glamorous advice but the difference between October and February on the cut is stark enough that it pays for itself fairly quickly in battery health alone.

What battery chemistry are you running? Lead-acid or lithium makes a difference to what I'd suggest next.

RhysPalmer | Posts: 847 | Location: Welsh Borders

@Rachel1995 yes, absolutely classic PWM behaviour in winter. The core issue is that PWM simply connects your panel directly to the battery once it's "full" — it can't harvest that extra voltage headroom that cold panels actually produce in abundance. Your panel is probably sitting at 19-20V on a crisp January morning but a PWM controller just clips that potential rather than converting it usefully.

For a narrowboat I'd seriously consider upgrading to even a budget MPPT — something like a Victron 75/15 sits around £60-70 and will genuinely transform your winter charging. You'll likely see 20-30% improvement when conditions are marginal.

Also worth checking your battery voltage at dusk — if you're consistently below 12.4V through winter, that's doing real long-term damage to lead-acid cells.

CrispySkipper | Posts: 312 | Location: Norfolk Broads

@Rachel1995 Worth mentioning something the others haven't touched on yet — your panel voltage matters a lot here. A 200W panel likely has a Vmp around 18-20V, and a PWM controller just clamps that straight down to battery voltage. You're essentially throwing away a significant chunk of potential charge.

In winter when you're already fighting low sun angles and short days, that inefficiency really bites. An MPPT controller would harvest considerably more — we're talking 20-30% gains in these conditions — by properly tracking the panel's optimal operating point.

Given your setup, I'd look at a decent 20A MPPT unit. Victron do a BlueSolar 75/15 for around £60-70, which would pay for itself pretty quickly over a UK winter. Might seem steep versus your £18 PWM but the difference on a boat is genuinely noticeable.

Been running a similar setup on my static caravan before I switched, and the penny finally dropped for me when I started actually logging the voltage my panels were producing versus what the PWM was delivering to the battery.

The core problem nobody's mentioned yet — PWM essentially clips your panel voltage down to battery voltage. So on a cold, clear January day when your panel might be pushing 19-20V open circuit, you're losing a huge chunk of that potential before it even counts.

Upgraded to a Victron SmartSolar MPPT 75/15 and the difference through winter was genuinely surprising. It harvests that extra voltage and converts it into usable current instead.

@Rachel1995 what's your battery bank — flooded lead acid or something newer? That affects how much this actually matters in practice.

LisaKelly | Posts: 203 | Location: West Yorkshire (off-grid cabin)

@Rachel1995 Something worth adding to what's already been said — have you checked your battery voltage in the mornings before any solar input? On my setup I noticed the overnight resting voltage was telling me far more about actual charge state than anything the PWM controller was reporting during the day. A fully charged lead-acid should sit around 12.7V after resting overnight. If you're consistently seeing 12.2V or below through December and January, your batteries are genuinely struggling rather than just the controller underperforming. Could be both issues compounding each other, especially on a narrowboat where you've presumably got heating and lighting loads running longer in winter. Worth ruling that out before spending on an MPPT upgrade, though I'll not pretend an MPPT wouldn't help regardless!