Is 200Ah of lithium enough for a 12V shepherd's hut through a UK winter?

Running a small Victron MPPT 75/15 with two 100W Renogy panels on my hut at the moment, feeding into a 200Ah Fogstar Drift LiFePO4 battery. Works brilliantly from April through to October, but I'm getting nervous about what happens when we hit November and the panels are barely generating anything useful.

The hut runs a 12V LED lighting circuit, a small 240V inverter for a laptop occasionally, and a diesel heater (Webasto clone) with its glow plug draw at startup. Reckon average daily consumption is somewhere around 40–50Ah in normal use, maybe higher if I'm working from the hut regularly.

Has anyone actually wintered a similar setup in the UK — particularly somewhere north of, say, Birmingham? Wondering whether I need to add a second battery, upsize the solar array, or just accept that a small 230V hookup or a battery-to-battery charger off a vehicle is going to be necessary once the days get short. What actually worked for you?

@12V_King what's your typical daily load in the hut? That's probably the key question before anyone can give a useful answer.

My gut feeling is that 200Ah might scrape through on mild winter days, but the real problem isn't the battery capacity — it's your solar input. Two 100W panels with a 75/15 controller in December/January in the UK? You're realistically looking at maybe 1-2 hours of decent equivalent sun, so perhaps 150-200Wh on a good day. That barely touches your available capacity.

Would you consider adding a small wind turbine or shore power backup for the darker months? Or even just upsizing to a third panel if the roof allows it.

Also worth checking — is your Fogstar rated for charging in sub-zero temps? Some LiFePO4 cells have built-in low-temp cutoffs that can catch people out over winter.

HamishTaylor87 | 47 posts

Good question from @StuKnight - load profile is everything here. But I'd also flag that your generation side is going to take a serious hit. Up in Scotland I see my panels producing maybe 15-20% of their rated output on a dull December day. Two 100W panels through a 75/15 controller could genuinely give you next to nothing for days on end during a cold snap.

200Ah LiFePO4 is decent capacity, but if you're heating the space at all (even a small mat or towel rail), that'll drain it surprisingly fast. What's your main heat source? If it's purely 12V loads - lighting, phone charging, maybe a small pump - you might scrape through. But I'd seriously consider a small backup charger from a generator or mains hookup just for the darker months. Belt and braces approach.

What @HamishTaylor87 is hinting at is probably the solar yield problem — in December across most of the UK you're realistically looking at 1–2 peak sun hours on a good day, sometimes less. Your 200W array might produce 60–80Wh on a grey January afternoon.

I ran a similar setup in my static caravan through last winter. Even with a modest load — LED lighting, phone charging, a 12V pump — I was regularly draining more than I was harvesting across three or four consecutive overcast days.

The Fogstar Drift is a solid battery, but 200Ah only gets you so far when input drops to almost nothing for a week. I ended up adding a small propane generator as a backup charging source rather than throwing more panels at it. Worth considering a shore power hookup or generator input via a Victron IP22 charger as a winter contingency.

@EcoFlow_Master is right on the yield issue, but nobody's mentioned the actual usable capacity hit in cold temps yet. LiFePO4 doesn't like charging below 0°C — Fogstar Drift has protection built in but that means your MPPT just sits there doing nothing on a frosty morning.

200Ah at 12V is 2.4kWh theoretical. Realistically you're using maybe 80% = ~1.9kWh. In December with a south-facing 200W array in the UK, you might scrape 0.5-0.8 kWh/day on a good day, nothing on overcast days.

If your daily load is more than ~150Wh you're going to be running a deficit pretty much constantly December-February without a hook-up or generator backup. What's actually drawing power in the hut — heating?

VDH_Boats | 203 posts

@RelayDream raises the cold capacity point which is the killer nobody warns you about. I learned this the hard way on the boat last January — my Fogstar Drift was sitting at around 80% rated capacity when temperatures dropped below 5°C for a week straight.

For a shepherd's hut I'd genuinely consider a small backup charging source alongside the solar. On my van build I ran a 240V shore power hookup for exactly these winter gaps. Even a modest 10A mains charger through a Victron Blue Smart would give you that safety net when three consecutive grey days in December drain you down to the buffer.

200Ah sounds generous until UK winter reality hits simultaneously from three angles — reduced yield, reduced usable capacity, and heating loads all conspiring at once.

Been through exactly this with my motorhome parked up over winter. What nobody's addressed yet is the consumption side — you can throw more panels and batteries at it, but if your heating, lighting and any 240V inverter loads aren't properly audited, you're just guessing.

Write out every load, its wattage, and realistic hours per day. December in the UK you're looking at maybe 1-2 usable solar hours on a good day, so your battery is doing the heavy lifting for 22+ hours.

200Ah is tight for a shepherd's hut with any meaningful heating. I'd want at least 300Ah and seriously consider a small mains trickle charger as backup — even a cheap Victron Blue Smart IP65 on a timer connected to a long extension lead from somewhere can save your bacon.

PVPro | 847 posts

Great thread. To build on what @ExChippie is touching on — have you actually logged your daily consumption yet? That's where most people get unstuck. A small oil-filled radiator even on the lowest setting will absolutely demolish a 200Ah bank overnight, regardless of temperature or solar yield.

For a shepherd's hut I'd be looking at a 12V diesel air heater (Webasto-style or the cheaper Chinese units) as your primary heat source rather than anything electric. Transforms the maths entirely. Keep the battery for lighting, phone charging, maybe a small 12V blanket — suddenly 200Ah looks much more manageable even through December.