Anyone know if a 200W panel is enough to keep a 12V 100Ah battery topped up over winter?

Running a small off-grid setup on my narrowboat and trying to work out if I'm pushing my luck heading into the colder months.

Currently have a single 200W panel (Renogy mono) feeding into a 100Ah lithium (Fogstar Drift) via a Victron MPPT. My daily consumption is roughly 30-40Ah — fridge, a few USB devices, LED lighting, that sort of thing.

In summer it's been absolutely fine, genuinely more than I need. But I'm getting a bit anxious about December/January. We're in the Midlands so not exactly the sunniest part of the world, and I know that usable sun hours up here can drop to maybe 1-2 hours on a bad day.

Quick rough maths tells me that even 1.5 peak sun hours would give me around 225-270Wh after losses — so maybe 18-22Ah at 12V. That's below my daily draw, which would mean the battery slowly depleting over a run of grey days.

A few questions for anyone who's been through a UK winter on similar kit:

• Is my maths roughly right or am I missing something?
• Does anyone have real-world data from a comparable setup?
• Would adding a second 100W panel make a meaningful difference, or is it more of a marginal gain when there's barely any sun anyway?

I do have the option to hook up to shore power a couple of times a week at some marinas, so it's not like I'd be completely reliant on solar — but I'd prefer not to depend on that.

Any experience gratefully received, especially from folks narrowboating or in similar cloudy/northern situations.

@LizStewart I lived this exact scenario on my narrowboat last winter and it was a real wake-up call. The panel wattage isn't really the issue — it's the hours. Come December you're looking at maybe 2-3 usable sun hours on a good day, often less when you're moored under trees or in a cutting. So realistically your 200W panel might only deliver 400-600Wh daily, if you're lucky.

The bigger question is what you're actually drawing. I was running LED lighting, a 12V pump, and phone charging — manageable. But the moment I added a diesel heater fan it got tight.

My honest advice: monitor your state of charge properly. A Victron BMV battery monitor was a game-changer for me — stopped me guessing and actually showed where the power was bleeding away.

Winter sun in the UK gives you roughly 1-2 peak sun hours on a good day, so that 200W panel is basically a 200-400Wh generator from November to February — fine if you're sipping power like a monk, but the moment you're running a diesel heater controller or any meaningful load you'll be in the red before Tuesday.

Really relevant question for me too — I've got a similar-ish setup in my static caravan in North Yorkshire and winter performance has been a proper eye-opener.

One thing worth adding to what @SaltyMechanic said about peak sun hours — the angle matters massively in winter. My panel is fixed mount and the low sun angle cuts into efficiency noticeably compared to summer. On a narrowboat you've presumably got some flexibility there?

Also worth checking what your actual daily consumption is in Wh before deciding if 200W is enough. Have you got a Victron BMV or similar battery monitor? That'll tell you exactly what you're drawing rather than guessing. I found I was using way more than I thought once I started monitoring properly.

What's your rough daily usage? That'd help figure out whether you need another panel or just tighter load management.

@LizStewart been there on my own boat — the brutal truth is UK winter will humble that panel faster than you can say "dead battery at 6pm"! 🌧️

Worth looking at your actual consumption first though — if you're being lean with a 12V setup, 100Ah of Fogstar lithium holds charge far better than lead-acid when it's sitting low over grey days.

Key things I'd add that nobody's mentioned yet:

• Panel angle matters massively in winter — steeper tilt chases that low sun
• A small Victron MPPT controller squeezes every last watt out of poor conditions
• Shore power hook-up as a backup isn't cheating, it's just sensible narrowboating

Second panel would honestly be my first move. Even a cheap 100W bolted alongside transforms your resilience.