Sizing a battery bank for a remote Scottish cabin — where did I go wrong with my calcs?

Finally got the shell of my wee off-grid cabin up near Loch Tay last autumn and I'm now trying to nail down the electrical system before I fit it out properly. I've been running the numbers on battery storage and I'm confusing myself, so thought I'd throw it out here.

My rough daily load is about 1.2kWh — LED lighting, a 12V compressor fridge, phone/laptop charging, and a small water pump. I was planning on 2 days of autonomy to cover the grey Scottish winters, which gives me 2.4kWh usable. I then divided by 0.8 to account for 80% DoD on lithium and landed on a 3kWh bank. That felt tight so I bumped it to a 5.12kWh 48V LiFePO4 setup using two 100Ah batteries in series.

Here's where I started second-guessing myself — someone on another forum said I should be planning for at least 4–5 days autonomy in Scotland during December/January, not 2. That would push me towards 8–10kWh, which is a completely different budget conversation. I've got two 200W panels going on the south-facing roof, so 400W total, which I know isn't going to do much on a dim January day in Perthshire.

Has anyone actually lived with a system like this through a Scottish winter? I'm trying to work out whether I'm massively underspec'd or whether the 4–5 day figure is just people being overly cautious. Would love to hear real-world numbers rather than theoretical ones.

WattJane | 847 posts | ⚡ Verified Contributor

@GE_Solar Congrats on getting the shell up — Loch Tay is a cracking spot!

One thing people consistently underestimate for Scottish cabins is the depth of winter. Your usable capacity drops significantly when temperatures are consistently near freezing, especially with lead-acid — you can lose 20-30% just from the cold. If you haven't already factored in temperature derating, that's often where the numbers go wrong.

Also worth checking: what days of autonomy are you designing for? Up that way in December you're looking at genuinely miserable solar windows, so I'd suggest a minimum of 3-4 days storage rather than the 2 days many calculators default to.

What battery chemistry are you considering and what's your estimated daily consumption? Happy to have a proper look at your figures.

Jock90 | 234 posts | 🚤 Narrowboat Nomad

@GE_Solar Near Loch Tay you say — factor in about 40 days of proper Scottish gloom between November and February where your panels produce roughly the same output as a slightly enthusiastic candle, so whatever your Victron MPPT is telling you to expect, halve it and then weep quietly into your Irn-Bru.

StormyNomad | 412 posts | 🏕️ Off-Grid Wanderer

@GE_Solar Great project! One thing folk consistently underestimate for Scottish Highland locations is the depth of discharge you can realistically achieve in winter. If you're using lithium you're fine pushing to 80-90% DoD, but if you've gone lead-acid, you really want to stay above 50% — effectively halving your usable capacity overnight.

Also worth checking: have you accounted for battery efficiency losses? Roughly 85-90% round-trip for lithium, less for lead-acid. People often just look at raw kWh figures and forget that charging and discharging both bleed capacity.

What chemistry did you go with, and what's your rough daily load estimate? Happy to cast an eye over your actual numbers if you share them — between @WattJane and @Jock90's local knowledge and a few of us running similar setups, we can probably spot where the calculations went sideways.