Shepherd's hut build - struggling to size my battery bank for winter, anyone done this?

Finally getting serious about the electrical side of my shepherd's hut after months of faff. The hut sits on a south-facing slope in Wales, which sounds ideal until November arrives and you're staring at four hours of usable sun on a good day. I've got a 400W Renogy panel array sorted but I genuinely can't work out how aggressive to be with battery capacity.

My daily load is pretty modest — LED lighting, a 12V compressor fridge, laptop charging, and the occasional power tool top-up. Running the numbers through a few calculators I keep landing somewhere between 150Ah and 300Ah at 12V, which is a fairly useless spread. Currently eyeing up a pair of Fogstar Drift 100Ah LiFePO4 batteries wired in parallel, so 200Ah usable. Wondering if that's optimistic for a Welsh winter.

The real question is whether anyone with a similar small-structure build has actually lived through a January or February on that kind of setup — not just modelled it. Calculators can't account for a week of solid grey cloud, which round here is basically a subscription service.

VanWez | 847 posts

Wales in winter is genuinely brutal for solar - I ran the numbers on my van build and was gobsmacked. You'll want to think less about panel wattage and more about days of autonomy, because you can easily get 4-5 consecutive days of proper gloomy weather up there.

Rough rule of thumb I use: calculate your daily Ah consumption, multiply by 5-6 for Welsh winter autonomy, then add 20% for battery inefficiency. Don't let anyone tell you 3 days is enough for your location.

What's your intended usage pattern? Weekends only changes the calculation massively compared to permanent living. Also worth knowing - are you wedded to AGM or open to lithium? The usable capacity difference is significant when you're already starting from a difficult position.

Tell us your rough daily loads and we can get more specific. 🙂

YorkshireExplorer | 847 posts | ⚡ Solar + Wind hybrid

@VoltWendy South-facing Wales is actually decent for solar but you're right to worry about November onwards - I'd say budget for roughly 3-4 days of autonomy minimum for a shepherd's hut sized setup.

The key question nobody asks early enough: what's your actual worst-case daily consumption? Lighting and phone charging is very different from running a compressor fridge.

Personally I'd suggest logging everything with a clamp meter before buying a single battery cell. I spent good money on a 200Ah bank thinking it was plenty, then discovered my diesel heater's control board was quietly sipping 8W continuously. Those phantom loads absolutely murder winter capacity.

What heating solution are you planning? That'll likely dictate your sizing more than anything else.

VoltageViking | 1,203 posts | 🚢 Narrowboat Life

On my narrowboat I learned the hard way that winter solar is basically decorative — budget for a decent Victron MPPT so at least your disappointment is efficiently managed, then size your Fogstar lithium bank for 3-4 days autonomy minimum because Welsh cloud doesn't take weekends off.

Megan1996 | 134 posts | 🌿 Garden Office

Running a garden office in similar conditions and honestly the thing nobody mentions is that usable capacity matters more than headline capacity. Even with a decent Fogstar Drift LiFePO4 bank I found I'd sized for average days but kept getting caught out by that miserable 3-day grey stretch you always get in January.

What loads are you actually running? My office is mostly laptop + lighting + a small heated pad and I still went bigger than I thought I needed. A shepherd's hut with heating involved is a different beast entirely.

If you haven't already, chuck your numbers into a Victron MPPT calculator — it won't account for Wales specifically but it gives you a baseline to then pessimistically double 😅

What's your roof space looking like? That's usually the real limiting factor before battery sizing even comes into it.

DodgySpanner | 2,341 posts | 🔧 DIY Solar Evangelist

@VoltWendy One thing worth flagging that I haven't seen mentioned yet - Wales gets absolutely battered with overcast days rather than outright darkness, so your panels will still generate something, just dramatically reduced. I'd plan around maybe 10-15% of rated output on a proper grey Welsh winter day rather than using zero as your worst case.

For sizing, work out your absolute bare minimum daily consumption first - lights, phone charging, maybe a 12V blanket - then build your bank to cover 3-4 days of that without any meaningful solar input at all. Lithium gives you that capacity without the weight penalty if you're moving the hut around.

What's your expected daily load looking like? That's really the starting point before anyone can sensibly suggest battery capacity.

KateMitchell73 | 847 posts | 🏡 Welsh Smallholder

@VoltWendy I'm literally just down the road from this situation - smallholding in Powys with a converted shepherd's hut we use as a guest space. What nobody told me before I sized mine was to track your actual December sun hours for your specific location rather than using generic UK averages. The difference between a sheltered valley and an open south-facing slope can be significant even within Wales.

I ended up with 400Ah at 12V (lithium) and genuinely wish I'd gone bigger for the winter months. Also worth considering a small backup charging option - even a modest wind turbine complement works well here because when the sun disappears, the wind usually doesn't. What's your rough daily consumption looking like? That'll help people give you more specific advice.

RogerJackson | 47 posts | ⚡ EV Charging

Something nobody's mentioned yet — are you planning any EV charging from the hut? Even a basic 3-pin outlet for overnight trickle charging will absolutely wreck your winter battery calculations. I made that mistake and had to completely rethink my bank size.

If EV charging is on your list, even occasionally, I'd strongly suggest sizing your Fogstar or similar LiFePO4 bank considerably larger than your "household" loads suggest, then adding it to your calculations separately. What looks like a comfortable 5kWh buffer disappears shockingly fast once a car is in the mix.

What are your actual consumption estimates for the hut itself? That'd help narrow down whether you're in the right ballpark before factoring anything else in.

Gazza82 | 1,156 posts | 🔆 Off-grid since 2019

@VoltWendy Wales in winter is brutal for solar - I'd seriously suggest modelling your worst case around maybe 0.8-1.2 peak sun hours per day for December/January rather than whatever the annual average says. That changes your battery sizing dramatically.

Rough rule of thumb I use: take your daily consumption, divide by your inverter efficiency (say 0.9), then multiply by however many cloudy days you want autonomy for. Four days minimum for Wales I'd say, possibly five.

What's your expected daily load looking like? Heating, lighting, devices? That's the starting point before any of us can give you sensible numbers.