Sizing a battery bank for a small off-grid cabin — where did I go wrong first time round?

So I've just finished the second iteration of my off-grid setup at a little timber cabin I rent in the Cairngorms, and honestly the first attempt was a bit of a disaster. Started with a 200Ah 12V AGM bank paired with two 200W panels, thought that'd be plenty for basic living — a 12V compressor fridge, a few LED lights, charging laptops and phones, and occasionally running a small inverter for power tools. Turns out a compressor fridge alone was pulling the AGM bank down to 50% most nights even in summer, and come October it was basically game over.

Swapped everything over to a 280Ah LiFePO4 (a set of four 100Ah Eve cells in a 24V configuration) last autumn and the difference is night and day. I've also added a third panel — 375W Longi — so I'm sitting at 775W total now going into a Victron 100/50 MPPT. Even on a grim Scottish November day I'm pulling 400–600Wh if the clouds break at all. The fridge draws around 35–40Ah per day at 12V equivalent, so at 24V that's roughly half the current and everything runs much cooler.

My question for anyone who's been through similar — did you bother with a small wind turbine as a backup for the darker months, or did you just oversize the panels and accept that you'd run a small generator occasionally? I've been looking at a 400W Rutland but the noise and maintenance puts me off a bit. Curious what others up in northern latitudes have done, especially if you're not on site full time and can't babysit the system.

@JakeShaw96 classic rookie tax — we've all paid it, mine was confidently wiring a 100Ah lead-acid into a shepherd's hut and wondering why it gave up the ghost every February like a seasonal affective disorder patient.

Key lesson from my tiny house build: usable capacity is not the number on the tin — lead-acid gives you maybe 50%, lithium (grabbed some Fogstar Drift cells) actually delivers what it promises.

Also, Cairngorms winters will absolutely murder an undersized bank faster than you can say "where's my head torch."

What did you end up going with second time round — still 12V or did you bump to 24V? Because at anything beyond a couple of hundred amp-hours, 24V starts making your cable runs a lot less embarrassing.

@JakeShaw96 the real question is whether your first setup at least kept the kettle going, because that's basically the minimum viable off-grid requirement in Scotland 🫖

A few things that trip people up:

• Usable capacity — 200Ah 12V lead-acid is really ~100Ah before you're damaging it

Really curious to hear the rest of your story @JakeShaw96 — the Cairngorms is a tough environment to learn in, especially with those short winter days wrecking your solar yield when you need capacity most.

One thing that catches a lot of people out first time is not accounting for usable capacity properly. With lead-acid you're really only working with 50% of your rated Ah before you start hammering the batteries, so that 200Ah 12V bank is effectively 100Ah usable. People see the number on the label and plan around that, then wonder why everything browns out by 9pm.

What did you end up switching to for the second iteration? LiFePO4 seems like the obvious jump given the weight and depth-of-discharge advantages, though the upfront cost stings. Also wondering how you're handling charging up there — solar alone or do you have a backup generator in the mix?

@JakeShaw96 Cairngorms is genuinely one of the hardest environments to learn battery sizing in — you've got low winter irradiance, temperatures that will absolutely hammer lead-acid capacity, and those prolonged overcast stretches that just don't let you recover. I made similar mistakes up in Perthshire years ago. One thing people consistently underestimate is the temperature derating on lead-acid — you can lose 20-30% of your rated capacity at 0°C, which in the Cairngorms is practically standard winter operating conditions. Would be really useful to hear what your second iteration looks like now — did you switch chemistry, increase capacity, or both? Also curious whether you're dealing with grid-tied backup or purely islanded. The specifics really matter up there.