Sizing a battery bank for a small off-grid cabin in Scotland — where do I even start?

Finally pulling the trigger on a wee cabin build up near Loch Lomond and I'm trying to get my head around storage sizing. I've been tinkering with solar on campervans for a few years so I'm not completely green, but a static cabin feels like a different beast altogether. The site gets decent summer sun but obviously Scottish winters are brutal — we're talking maybe 1–2 peak sun hours on a bad January day.

My rough daily load is around 1.5–2 kWh. That covers LED lighting, a 12V compressor fridge (about 40Ah/day), a laptop, phone charging, and occasional use of a small inverter for power tools. I'm planning a 24V system and leaning towards LiFePO4, probably 200Ah to start, which gives me 4.8 kWh usable at 100% DoD — though I know you don't want to push it that hard regularly.

The part I'm struggling with is working out how many days of autonomy I should realistically plan for in winter without the panels doing much. Do people up here tend to pair their solar with a backup generator or a wind turbine to cover those grey weeks? I've seen a few builds using small Victron MPPT controllers and a Multiplus for generator integration, which looks tidy but the cost adds up quickly.

Has anyone built something similar in a high-latitude, high-rainfall climate and can share what their actual winter experience has been like? Keen to hear what worked and what you'd do differently.

MoorClive | 847 posts | Stirlingshire

@DanFisher89 Brilliant project, and Loch Lomond is stunning — though that'll work against you! Scotland's irradiance is genuinely poor November through February, so I'd strongly recommend sizing your battery bank around winter autonomy rather than peak summer output. Work out your daily consumption in Wh (be honest, add 20% contingency), then decide how many days of autonomy you want without meaningful solar input — I'd suggest 3-4 days minimum up there. Multiply those together and that's your rough usable capacity target. Don't forget to account for your battery chemistry's depth of discharge — LiFePO4 gives you ~80-90% usable versus ~50% for lead-acid. What's your expected daily load looking like? That's really the starting point before anything else.

BoxerProject | 312 posts | Array

@DanFisher89 Scotland in winter — where the sun sets at 3pm, rises at 9am, and spends the six hours in between hiding behind a cloud shaped suspiciously like another cloud.

Seriously though, the key number nobody tells you upfront: **multiply your daily consumption by at

DalesCruiser | 203 posts | Array

@DanFisher89 Scotland's brutal for winter solar tbh — I'd honestly size your battery bank way bigger than the calculators say. Did my shepherd's hut last year and massively underestimated the consecutive grey days problem.

Work out your daily Wh, then multiply by however many days you want autonomy without sun. Scotland? I'd say minimum 4-5 days. Then add ~20% buffer for depth-of-discharge headroom.

Fogstar Drift LiFePO4 cells are decent value if you're comfortable building a pack. Otherwise Victron/Pylontech if you want plug-and-play. The Victron SmartShunt is worth every penny for actually knowing your state of charge rather than guessing.

What's your likely daily load looking like? That's really where to start before any of the rest makes sense.

MrBodge73 | 156 posts | Array

@DanFisher89 Built my garden office on similar assumptions and nearly froze in the dark — pro tip: whatever number you land on, double it, then add a Victron MultiPlus because Scotland will laugh at your spreadsheet come February.

T5Build | 847 posts | ⚡ Off-Grid Enthusiast

@DanFisher89 Good news is your campervan experience gives you a solid foundation — you already understand the basics, you just need to scale up the thinking.

One thing nobody's mentioned yet: don't just calculate from your load side, also work backwards from your worst-case generation window. In Scotland that can genuinely be 5-7 consecutive days of heavily overcast skies in December/January with barely a usable sun hour. Your battery bank needs to bridge those gaps, not just overnight.

Practically speaking, list out every load with realistic daily hours, then multiply by at least 4-5 days autonomy. Factor in a 50% depth of discharge limit if you're going lithium, less if LiFePO4.

What's your planned generation mix? Wind alongside solar makes a huge difference up there — worth considering seriously.

OakSpirit | 412 posts | 🔋 Battery Nerd

Scotland's day length in December is the killer variable people underestimate. Loch Lomond sits around 56°N — you're looking at maybe 1-2 peak sun hours on a bad winter day, not the 3-4 you might've used for van calculations.

Start with your daily Wh load, then work backwards:

• Divide by 0.5 (don't drain LiFePO4 below 50% if you want longevity, 80% DoD absolute max)
• Multiply by your autonomy days (3-5 minimum for Scotland winters)
• Add ~20% for inverter/cable losses

Fogstar Drift cells are decent value if you're DIY building a bank. Victron SmartShunt is non-negotiable for proper state-of-charge monitoring — my narrowboat taught me that lesson the hard way.

What's your expected load list? Heating type matters massively here.