Sizing a battery bank for a remote cabin — how much is enough for winter?

Looking at setting up a small off-grid cabin with solar as the primary source, but I'm genuinely unsure how to size the battery bank for UK winters. Days are short, skies are grey for weeks on end, and I don't want to be caught out. On my boat I run a 200Ah lithium (Fogstar Drift) with 400W of panels and it's fine for most of spring/autumn, but a cabin feels like a different beast altogether.

The load I'm planning for is fairly modest — LED lighting, a 12V fridge, phone/laptop charging, and maybe a small inverter for the odd power tool. I've roughed out about 1.5–2kWh of daily consumption. The question is whether I should be sizing the battery for, say, 3 days of autonomy or pushing towards 5–7 given how grim January can get in the UK.

Has anyone gone through the process of sizing specifically for winter in Britain? I keep seeing advice based on sunny climates and it just doesn't translate. Also wondering whether it's worth adding a small generator input (thinking a Victron Multiplus for the hybrid charging side) rather than just going pure solar and hoping for the best.

ChrisCampbell90 | 847 posts

@Taffy great question and honestly one I wrestled with for ages before my Welsh borders cabin build.

The bit people consistently underestimate is consecutive grey days — not just short days. I'd plan for at least 5-7 days of autonomy in winter, not the 2-3 days often suggested for sunnier climates.

Practically speaking, audit your actual loads ruthlessly first. Lighting, a 12v fridge, and phone charging is very different to running a kettle regularly.

Also worth considering a small backup generator or wind turbine as a hybrid approach rather than just throwing more batteries at the problem. A 800W genny running 2 hours charges a decent bank without needing absolutely enormous capacity.

What's your rough load estimate looking like? That'll determine everything really before recommending specific kWh figures.

MoorDweller | 1,203 posts

@Taffy the thing people often underestimate is consecutive low-yield days rather than just total capacity. I'd plan for at least 5–7 days of autonomy without meaningful solar input — that's your real design target for a Dartmoor or Scottish winter scenario.

Also worth factoring in that your panels will be operating at a much shallower angle to the sun, so even on clear winter days you're getting a fraction of summer output. Don't just look at panel wattage, look at actual December/January yield data for your specific latitude using PVGIS.

A backup source — even a small wind turbine or a generator for occasional top-ups — transforms the maths considerably and lets you size the battery bank more sensibly without going completely overboard. What's the cabin's rough daily consumption looking like? That's really where the conversation needs to start.

BoatMick | 312 posts

@Taffy worth running the numbers properly rather than guessing — I sized my shepherd's hut bank at what I thought was generous and still got caught out second winter.

Rule of thumb I now use: calculate your daily Wh consumption, then multiply by 5-7 days of autonomy for UK winter rather than the 2-3 days you see in most generic guides. Grey November weeks are no joke.

Also consider your Depth of Discharge carefully — if you're going lithium (Fogstar Drift cells are decent value), you can push to 80-90% DoD. Lead acid, keep it above 50% or you'll knacker the bank within a couple of seasons.

What's your rough daily load looking like? Heating, lighting, devices? That changes the answer considerably.