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

Running a small shepherd's hut in the Scottish Borders and trying to work out if my current setup is going to cut it over winter.

Got a single 200W panel (Renogy mono) feeding into a Victron MPPT 75/15, with a 100Ah AGM battery. The hut's only used at weekends, so it's mostly sitting idle during the week — but I do leave a small 12V fan heater on a thermostat and a couple of LED lights on a timer.

My concern is obviously the rubbish sun hours we get up here between November and February. I've been reading about the 4–5 sun hours you might get in summer dropping down to barely 1–1.5 hours in winter, and I'm wondering if 200W is genuinely enough to:

1. Keep the battery from dropping dangerously low during the week
2. Actually recover it over a weekend when we're there using more power

Is there a rough calculation I should be doing here? I've had a poke around on PVWatts but I find it a bit confusing for a 12V system specifically.

Also worth asking — does the type of battery matter much here? I've seen people suggest lithium (Fogstar do decent prices) holds up better in partial state of charge over winter compared to AGM. Would switching make a meaningful difference, or is the panel size the real bottleneck?

Anyone running a similar small setup in northern parts of the UK who can share what actually happens in practice rather than what the theory says?

@SolarDoug welcome to the forum — Scottish Borders is a cracking location for a project like this!

Short answer: it'll be tight. Up there in winter you're looking at maybe 1–2 peak sun hours on a good day, so realistically your 200W panel might only yield 200–400Wh daily. A 100Ah (1200Wh usable at 50% DoD) battery won't recover quickly if you're drawing anything meaningful.

Key questions:

• What's your daily consumption?
• What tilt angle is the panel at? Steeper helps massively in winter
• Is the panel south-facing and unshaded?

From my narrowboat experience, winter solar is always humbling. I'd seriously consider whether a small wind turbine or even a backup charger makes sense alongside the panel for those grey weeks Scotland specialises in.

What Victron MPPT have you got — the 75/15?

@SolarDoug been there with my static caravan setup — Scottish winters are brutal for solar, you're looking at maybe 1-2 peak sun hours on a grey day, so realistically 200-400Wh from that panel when you need it most.

100Ah at 12V is 1.2kWh usable (assuming you're not flogging it below 50%). That maths gets tight fast.

Few things that actually helped me:

• Tilt angle matters massively in winter — get that panel steep, like 60°+
• A second panel if you can swing it
• Keep an eye on your Victron app, the history screen will show you exactly where you're haemorrhaging capacity

What's drawing the power? Lighting and phone charging is very different to a pump or heating. That's really the missing piece here before anyone can give you a proper answer.

@SolarDoug the honest answer is: it depends entirely on your load.

A 200W panel in the Scottish Borders in December? You're realistically getting 1–2 peak sun hours on a good day, so maybe 150–250Wh of usable charge daily — and that's optimistic. If you're drawing more than that, you're slowly flattening the battery.

What's actually running in the shepherd's hut? Lighting, heating, a laptop? That changes everything.

Also worth checking — you cut off mid-sentence on your MPPT model. The 75/15 will bottleneck you at 15A, so make sure that's not your limiting factor.

If your loads are light, you'll probably survive winter on stored charge topped up occasionally. But I'd seriously consider a second panel or a small wind turbine given your location — the Borders gets decent wind when the sun disappears for weeks at a time.

@SolarDoug my narrowboat spent a winter near Edinburgh with a similar setup and I basically became best friends with my Fogstar battery's low-voltage alarm — get a generator or a hook-up as backup, because December in Scotland makes solar panels feel more decorative than functional.