Narrowboat solar setup – struggling to keep batteries topped up on cloudy days

I've been running a 200W panel on my narrowboat for about 18 months now, paired with a Victron SmartSolar 100/20 MPPT and a pair of 110Ah leisure batteries (lead-acid, sealed). On decent days it's brilliant – I'm pulling 8-10A easy and the batteries are sitting happy. But lately, especially mooring up under tree cover or during our delightful British grey winters, I'm barely seeing 2-3A and the batteries are dragging down to 11.8V overnight if I'm running the inverter for much at all.

I'm starting to wonder whether I should just bite the bullet and add another 200W panel. The roof space is there – I've got roughly 1.2m x 2m free on the stern end. The complication is the tiller arm swings through part of that space, so I'd probably need to split it into two smaller panels rather than one big one. Has anyone done a split-panel arrangement like that on a narrowboat and managed to wire them sensibly without losing too much efficiency?

The other thing I'm mulling over is whether the lead-acid batteries are just the wrong tool here. I've seen a lot of chat about dropping in a 100Ah lithium (LiFePO4) to replace the pair, which would give me genuinely usable capacity rather than the 50% I'm babying out of the lead-acids. Budget's tight though – ideally keeping the whole upgrade under £400 if that's even realistic.

Would love to hear from anyone who's running solar on a narrowboat or similar, particularly if you've dealt with the shading problem or made the switch to lithium on a tight budget. What actually made the biggest difference for you day-to-day?

Reply by PeteGreen84:

@OffGridMick Totally understand the frustration – cloudy UK days are brutal for lead-acid, especially since you really shouldn't be pulling them below 50% without killing the lifespan sharpish.

A couple of thoughts: 200W is honestly quite modest for full-time liveaboard use. If you can squeeze another 100-200W on the roof, that'll make a noticeable difference even in overcast conditions since panels still generate something from diffuse light.

Also worth checking your battery wiring – loose connections or undersized cable can cause significant losses that you'd never notice on sunny days but really show up when every watt counts.

Long-term, if budget allows, lithium would transform your setup. You'd effectively double your usable capacity overnight without adding a single extra panel. Victron kit plays nicely with LiFePO4 too, so your MPPT wouldn't need replacing. 🙂

@OffGridMick Have you looked at what your actual daily consumption is versus what that 200W panel realistically generates in the UK? Even in summer you're probably averaging 2-3 peak sun hours on a good day — that's 400-600Wh at best. Lead-acid also has that nasty absorption phase that drags on forever when you're only partially topped up.

Curious — are you running an EV or any kind of high-draw charging loads on there? That would change the conversation significantly. If not, even upgrading to a single 200Ah LiFePO4 (Fogstar do decent cells) would transform your usable capacity on grey days since you can actually pull the full 80%+ without damaging them.

What's your heaviest draw on board day-to-day?

@OffGridMick worth checking whether your MPPT settings are actually configured correctly for sealed lead-acid — the absorption and float voltages vary quite a bit depending on the battery type. Got caught out with this on my own setup and was essentially undercharging for months without realising.

Also, have you considered that 220Ah of lead-acid effectively only gives you ~110Ah usable before you start damaging them? If your consumption is anywhere close to that, you're going to struggle regardless of how much sun you get.

What's your typical daily load? Running a 12V fridge, lighting, inverter? That'd help narrow down whether this is a generation problem or a consumption problem — they need quite different solutions.