Anyone know if a 200W panel will keep a 100Ah leisure battery topped up over winter?

Running a narrowboat over winter and trying to work out if my setup is going to be viable without running the engine every couple of days just to top things up.

Currently got a single 100Ah AGM leisure battery (Varta, nothing fancy) and I'm looking at adding a 200W panel on the cabin roof. Usage is fairly modest — LED lighting, a 12V compressor fridge, phone/laptop charging, and a small water pump. Probably 40-50Ah per day at a rough estimate.

My concern is the winter irradiance figures for Yorkshire. I've been using the PVGIS tool and getting peak sun hours somewhere around 1.0–1.5 in December/January. That would suggest maybe 200–300Wh on a good day, so around 16–25Ah — which clearly doesn't cover the 40-50Ah I'm pulling.

A few questions:

• Is my PVGIS reading method correct, or am I doing something wrong?
• Would upgrading to a 200Ah battery just delay the inevitable, or does it actually help meaningfully over a week of poor weather?
• Has anyone run a similar setup on a narrowboat and managed it without constant engine running?

I'm trying to avoid going diesel genny if I can help it. The mooring situation also changes as we move, so panel angle isn't always ideal — though I could potentially tilt it manually.

Wondering if I should just bite the bullet and look at a second panel rather than a bigger battery. Space on the roof isn't unlimited but there's probably room for another 100W at a push.

Compo_7207 | Posts: 847

@YorkshireVanLifer Short answer: probably not on its own, I'm afraid. A 200W panel sounds decent but winter in Yorkshire is brutal for solar - you're realistically looking at maybe 1-2 peak sun hours on a good day, often less. That's potentially only 200-400Wh coming in daily, assuming your MPPT is doing its job properly.

The bigger question is what your daily consumption actually looks like. If you're running a 12V fridge, LED lighting and charging devices, you could easily be pulling 50-80Ah per day in winter with heating fans etc.

A narrowboat has the advantage of being able to run the engine for charging obviously, but I'd also seriously consider a small wind turbine as a supplement - canals can be surprisingly breezy and wind generates overnight when solar can't. What charge controller are you running currently?

DevonDweller | Posts: 2,341

@Compo_7207 is right to be cautious. Worth crunching the actual numbers though — in December/January you're realistically looking at 1–2 peak sun hours per day in Yorkshire, so that 200W panel might yield 200–400Wh on a decent day. If you're drawing more than that (heating controls, lighting, phone charging, water pump), you'll be in deficit daily.

The deeper problem is AGMs really dislike sitting at partial state-of-charge over winter — sulphation becomes a genuine concern.

My honest recommendation: consider adding a Victron SmartSolar MPPT if you haven't already (even a cheap Renogy one will help), and look at a battery-to-battery charger off your engine alternator to make those engine runs count properly rather than relying on a basic split-charge relay.

What's your actual daily consumption looking like? That's the critical figure here.

AndyGraham | Posts: 1,156

@YorkshireVanLifer The thing that'll really bite you isn't just low irradiance — it's the panel angle. A narrowboat roof is essentially flat, which is brutal in December when the sun barely clears 15° elevation. You're losing another 30-40% output on top of the already grim ~0.8 kWh/m²/day figures for Yorkshire in midwinter.

My shepherd's hut setup taught me this the hard way. Switched to a tilted mounting bracket and gained a noticeable difference.

Also worth noting: AGM batteries genuinely don't like sitting at partial state of charge for extended periods — sulphation becomes a real issue. A Victron SmartSolar with the proper AGM absorption/float profile will help protect the battery on the days you do get decent sun.

Realistically though, consider a small DC-DC charger wired to the engine alternator as your primary source, with the panel handling maintenance duty.

BatteryWez | Posts: 412

@YorkshireVanLifer One thing nobody's mentioned yet — AGM chemistry is particularly brutal in winter. You'll struggle to push a proper full charge into it when the battery itself is cold, and if you're regularly sitting at 50-60% SOC it'll sulphate faster than you'd think.

Honestly the single biggest upgrade you could make before worrying about adding more panels is fitting a Victron BMV-712 so you actually know what's going on. Flying blind with a leisure battery on a boat in December is how you kill it by February.

I run LiFePO4 in my shepherd's hut now and the winter difference vs my old AGM is genuinely night and day — but if budget doesn't stretch, at least protect what you've got with proper monitoring first.