Tiny cabin build in the Scottish Borders — what size battery bank am I actually looking at?

Right, so I'm finally cracking on with a wee off-grid cabin project I've been planning for about two years. It's a 20-foot timber-framed structure up near Jedburgh, fairly remote, no grid connection within a sensible distance. The plan is to use it as a weekend retreat mostly, with occasional longer stays in summer — probably 3 to 5 days at a stretch.

In terms of loads, I'm trying to keep it genuinely minimal. We're talking LED lighting throughout (maybe 150W peak total across six or seven fittings), a 12V compressor fridge (the Alpicool CF55 — pulls around 45W when running), a small inverter for charging laptops and phones, and a Webasto diesel heater which I know sips a fair bit on startup. I've got two 200W panels earmarked already but I'm well aware Scotland in November isn't exactly the Algarve.

What's tripping me up is sizing the battery bank. I've been looking at 200Ah of lithium (so two 100Ah 12V LiFePO4s in parallel) but I genuinely don't know if that's going to be enough buffer for a run of grey days — which up in the Borders is basically the default setting from October through March. Do I need to be thinking 400Ah minimum, or am I overthinking it and the panels will cope if I'm sensible with usage?

Has anyone run something similar in Scotland or northern England where the winter irradiance is properly miserable? Really curious what real-world battery sizing looked like for you rather than what the calculators spit out.

@DaiWalker61 Scottish Borders is going to hammer you in winter — short days, low sun angle, lots of overcast. Don't undersize the panels thinking summer performance is your baseline.

Key question before anyone can give you a sensible battery figure: what's your actual load list? EV charging especially — that changes everything. A small domestic cabin might scrape by with 5-10kWh usable storage, but chuck in even slow EV charging and you're looking at 20kWh+ pretty sharpish.

Personally running Victron kit with Fogstar Drift lithium — solid combo. Victron's sizing tools are worth using early doors.

Also — how many days autonomy are you planning for? Jedburgh can go a week of proper grim weather easy.

@DaiWalker61 I built out a shepherd's hut on a similar latitude and the winter performance genuinely humbled me. What nobody told me until I'd already sized everything was the concept of usable days — in December you might get two or three genuinely productive solar days in a row, then a week of grey nothing.

The battery bank isn't just about daily consumption, it's about how many days of autonomy you want without the panels doing much. I run a Victron SmartShunt and watching those logs through January was an education.

Rough rule of thumb I settled on: calculate your daily consumption, multiply by four to five days autonomy, then factor in that you should only drain lithium to about 20% SOC. Fogstar Drift cells gave me good capacity-per-pound when I was speccing mine.

What's your expected daily draw looking like?

@DaiWalker61 Rule of thumb I wish someone had tattooed on my hand before I built out my shepherd's hut: double whatever bank size you think you need, then add 20% — and that's before you've even looked at a DNI map for Jedburgh in December, which will make you want to cry into your Victron app.

@FETFan tattooed on your hand sounds painful, but probably still cheaper than buying undersized batteries twice like I did on the narrowboat 😂 — Fogstar Drift 200Ah felt massive until January arrived and I was rationing the kettle like it was wartime.

@DaiWalker61 Worth flagging that Jedburgh specifically sits in quite a sheltered valley — the hills around it can cast shadows earlier than you'd expect in winter, so factor that into your panel orientation planning rather than just latitude alone. I'd also strongly suggest getting a year's worth of hourly irradiance data from PVGIS for your exact location before sizing anything. Plug your estimated daily consumption in and let it calculate worst-case monthly figures. The December and January numbers will probably make your eyes water a bit, but better to know now. What's your heating situation — are you planning to keep that entirely off the electrical system? That decision alone will dramatically change what you're looking at for battery capacity.

@DaiWalker61 Before anyone can give you sensible numbers, it'd really help to know your expected daily consumption in watt-hours and how many days of autonomy you're planning for — i.e. how long do you need to run without meaningful solar input? Up near Jedburgh you could realistically see 4-5 consecutive gloomy days in winter, so that autonomy figure matters a lot.

Also, are you heating via electric or wood burner? That single decision can swing your battery requirements enormously. Most cabin builds I've seen go wrong because they try to run an immersion heater or electric radiators off batteries — that's a wood burner's job in Scotland, full stop. Keep your electrical loads to lighting, devices, and maybe a 12v fridge, and your battery bank becomes far more manageable.

What's your rough load list looking like so far?