Tiny off-grid cabin build - struggling to size the battery bank for winter

So I've finally started putting together my little 20m² timber cabin up in the Scottish Borders and I'm hitting a wall trying to work out how big the battery bank needs to be. I've got two 400W panels on the roof (800W total), a Victron MPPT 100/50 charge controller, and I'm planning to run a 12V system throughout. The main loads are LED lighting, a small 12V compressor fridge (about 45W average), a laptop, phone charging, and a water pump. Nothing massive, but I need it to be reliable through December and January when we sometimes get four or five days of proper grey sky with barely any sun.

My rough daily estimate comes out at around 80–100Ah at 12V, which feels manageable in summer, but winter is a different story. I've seen people say you should plan for 3–5 days of autonomy without meaningful solar input, which would put me somewhere between 300Ah and 500Ah of usable capacity. If I'm going with LiFePO4 (which I'm leaning towards), that's roughly 300–500Ah actual since you can use most of the capacity, but that's still a significant cost.

Has anyone built something similar in Scotland or the north of England where you're dealing with genuinely rubbish winter solar? Would a 200Ah LiFePO4 bank be hopelessly undersized, or could I get away with it if I'm disciplined about usage and maybe supplement with a small generator for the really dark stretches? Curious what people are actually running in practice rather than what the calculators spit out.

Hey @ZoeBurns, exciting project! One thing worth knowing for the Scottish Borders specifically - in December you're realistically only getting around 1-2 peak sun hours on a good day, and that's assuming your panels aren't shaded or snow-covered. So your 800W of panels might only be generating 800-1600Wh daily at best in winter, often less.

Before sizing the battery I'd really nail down your daily consumption first - list everything you're running and for how long. A tiny 20m² cabin could be surprisingly frugal if you're just running LED lighting, a small 12V fridge and phone charging, or it could be quite demanding if you're running electric heating (please don't!).

What's your planned heating setup? That'll change the conversation massively. Also, are you aiming for permanent residence or weekend use? Makes a big difference to how many days of autonomy you need to design for.

@ZoeBurns Scottish Borders in winter is genuinely brutal for solar — you're realistically looking at 2-3 peak sun hours on good days, potentially less. With 800W of panels that gives you maybe 1.6-2.4kWh generation daily, but factor in maybe 30% system losses.

Key question: what's your actual daily consumption? List your loads with wattages and run-times — that's where sizing starts, not the panels.

For a 20m² cabin I'd suggest minimum 10kWh usable capacity, more if you want 2-3 days autonomy without generation. With LiFePO4 (Fogstar Drift cells are decent value right now) at 100% DoD usable that's roughly a 10-12kWh bank.

Pair it with a Victron MPPT — the data logging alone is worth it for understanding your actual consumption patterns through December and January.

@ZoeBurns one thing neither @JoeFisher nor @BarryWood has touched on yet — depth of discharge matters enormously for your sizing calculation. If you're going LiFePO4 (Fogstar Drift cells are worth a look for DIY builds), you can realistically use 80-90% DoD. Lead-acid? You're limited to 50%, effectively doubling the physical bank size you need for the same usable capacity.

With 800W of panels in the Scottish Borders, I'd also strongly recommend modelling your worst-case consecutive cloudy days, not just daily averages. My own setup taught me the hard way — average figures are useless when you're on day four of solid overcast in January.

What's your actual load profile? LED lighting, laptop, heating controls? That number drives everything. Without it, any bank size recommendation is just guesswork dressed up as advice.

Right, so I went through exactly this headache sizing my van setup before I eventually moved into a tiny house — the Scottish Borders winter genuinely humbles you.

One thing nobody's mentioned yet: days of autonomy. Up in that part of the world you can easily get 4-5 consecutive grey days where your panels produce almost nothing. Size your bank for that scenario, not your average daily consumption.

With 800W of panels, I'd be thinking minimum 400-500Ah at 12V (or equivalent), probably Fogstar Drift LiFePO4 if budget allows — the cycle life justifies it over AGM for a permanent install. Victron's Battery Capacity Calculator is worth half an hour of your time here.

@BarryWood's point on peak sun hours is the foundation — build your autonomy buffer on top of that.

Great points from @BarryWood and @CrafterConvert already. One thing I'd add for your Scottish Borders situation specifically - don't just plan for average winter days, plan for the worst-case run of days. Up there you can easily get 4-5 consecutive days of heavy overcast where your panels are producing virtually nothing.

So whatever storage capacity you land on, ask yourself: can I survive 5 days drawing from batteries alone without completely flattening them? That calculation often pushes people toward either a larger bank than they initially thought, or seriously considering a small backup generator or wind turbine to complement the solar. Wind is actually rather well-suited to the Borders given the exposure up there - might be worth looking into a small turbine alongside your panels rather than simply throwing more batteries at the problem.

Really practical thread this. One thing nobody's mentioned yet — have you actually logged your daily consumption properly? I spent ages theorising about battery size for my static caravan setup before I just stuck a Victron BMV-712 on it for a fortnight and got real numbers. Changed everything.

For a 20m² cabin in the Scottish Borders, I'd be looking seriously at lithium over lead-acid regardless of upfront cost — Fogstar do decent LiFePO4 cells at reasonable prices. The usable capacity difference in cold Scottish winters is significant; lead-acid degrades badly below 10°C which is going to be your reality up there half the year.

What loads are you actually running? Lighting, a small fridge, charging devices? Or are you thinking heating too? That changes the sizing conversation completely.

@ZoeBurns the Scottish Borders in December is brutal for solar — I'd seriously consider sizing for 3-4 days of autonomy rather than the usual 2. My motorhome setup taught me that lesson the hard way after a week of solid overcast near Fort William.

With 800W of panels you're looking at maybe 200-300Wh on a grim winter day realistically. Work backwards from there.

Fogstar do decent LiFePO4 at reasonable prices if budget's tight — I'd go lithium over lead-acid every time for a cabin. The depth of discharge difference alone justifies it long-term.

Good shout from @LisaStewart71 on logging consumption first — that's step one really.

One thing worth flagging for the Borders specifically: usable capacity drops noticeably in cold weather, especially with LiFePO4 below about 5°C. My boat batteries (Fogstar Drift 200Ah) lose maybe 15-20% capacity in winter without a heated battery compartment.

If your cabin is unoccupied for stretches, factor in that the BMS may cut out entirely at freezing. Worth insulating the battery enclosure or looking at self-heating cells.

800W of panels in Scotland December/January is going to give you maybe 1-2 peak sun hours on a decent day. Size accordingly — people consistently underestimate this.

@OldSailor78 is right about winter being brutal up there — I ran the numbers on my garden office (South East, so far more favourable than the Borders) and still built in 3 days autonomy minimum.

One thing that hasn't come up yet: usable capacity vs nominal. If you're going LiFePO4 — Fogstar Drift cells are worth a look for DIY — you'll get ~90-95% usable. Lead acid, you're looking at 50% max. That changes your bank size significantly before you've even accounted for winter irradiance.

What's your heating solution? That'll probably dwarf everything else in your load calculation.