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

Running a small off-grid setup at my allotment shed — 100Ah AGM leisure battery with a Renogy 20A MPPT controller. Currently got a single 200W panel on the roof.

Works brilliantly through summer obviously, but I'm starting to worry about the winter months. Here in the UK we're looking at what, maybe 1–2 peak sun hours on a good day in December/January? So realistically I'd be pulling maybe 200–400Wh on a clear day at best.

The shed draws power for:

• LED lighting (around 20W)
• A small 12V fan heater occasionally
• Charging tools and phones

My main concern isn't running everything flat out — it's whether the panel can at least keep the battery healthy during the weeks where I'm barely using it. I've read that AGMs really don't like sitting at partial state of charge for extended periods.

Has anyone actually monitored their winter solar yield in the UK with a similar setup? I'm using a basic Victron BMV-712 so I can track what's going in and out properly.

Also wondering whether it's worth adding a second panel — maybe a flexible 100W one on a south-facing tilt mount — or if that's overkill just for maintenance charging. Budget isn't massive so trying to avoid spending unnecessarily.

Is 200W genuinely sufficient for keeping a 100Ah battery ticked over through a British winter, or am I kidding myself?

@SparkyHiker honest answer — probably not on its own. Winter UK solar is rough, you're realistically looking at maybe 1-2 peak sun hours on a grey December day, so your 200W panel might only yield 200-400Wh on a good day.

If your allotment shed is light usage (LED lights, phone charging etc) you might scrape by. But I had a similar setup in my garden office and the battery was constantly sulking by January.

Few options:

• Tilt your panel steeper (~60°) for winter sun angle — made a noticeable difference for me
• Add a second panel if the roof allows
• Consider a small trickle charger as backup

Also AGMs really don't like sitting discharged — kills them fast. Keep an eye on voltage over winter if nothing else! 🔋

@CrispyWelder is on the right track but worth adding some numbers.

UK winter insolation can drop to 0.5–1.5 peak sun hours depending on latitude and cloud cover. So your 200W panel is realistically producing 100–300Wh on a decent day — and that's before MPPT losses and battery acceptance rate.

If your shed load is minimal (few LEDs, maybe a phone charger) you might scrape by. But an AGM sitting at partial state of charge repeatedly through winter will sulfate and you'll kill it within a couple of seasons.

My recommendation: either add a second panel in parallel or — and I'd actually push this harder — switch to LiFePO4. A Fogstar Drift 100Ah will handle partial SoC cycling far better than any AGM without the sulphation penalty.

What's your actual daily load? That's the number that matters here.

@CrispyWelder and @WhatsAFuse65 have covered the solar maths well, so I'll add the bit nobody's mentioned yet — self-discharge and parasitic loads.

AGMs are notorious for this. Even sitting idle over winter, that 100Ah is slowly draining. Chuck in any controller standby draw and you're fighting a losing battle on a cloudy January fortnight.

Ran into exactly this with my motorhome setup before I switched to LiFePO4. The panel technically produced enough — the battery just couldn't hold what it received efficiently enough.

Two practical fixes worth considering:

• Add a trickle/maintenance mode on your Renogy if it has one
• Seriously consider whether a small second panel (even a cheap 100W Renogy) would give you the redundancy margin you need

Winter UK solar is brutal. Margin isn't a luxury, it's the whole game.

Really useful thread — this is exactly the kind of thing I wrestled with before setting up my tiny house system.

One thing nobody's touched on yet: panel angle matters enormously in winter. If your panel is flat on a shed roof, you're leaving a significant chunk of that already-limited winter generation on the table.

In the UK, tilting to around 60–70° in winter vs the typical summer 30–35° can make a noticeable difference to daily yield. Worth checking if your Renogy mount allows seasonal adjustment.

Also — what's your actual load? A 100Ah AGM at 50% usable depth means you've got ~50Ah to play with. If your shed draw is minimal (a few LEDs, maybe a phone charge), a 200W panel might just scrape through on decent winter days.