Struggling to size my battery bank for a off-grid cabin — am I overcomplicating this?

Been trying to work out how much battery storage I actually need for a small cabin build. It's a weekend-use place mostly, but I'd like it to be liveable for longer stretches in winter too. Currently thinking 2–3 days of autonomy without sun, which in the UK feels essential given the weather.

My rough load calc comes to about 1.8kWh/day — LED lighting, a 12V compressor fridge, laptop, phone charging, and a small water pump. I've got a 400W solar array planned (4× 100W panels, probably Renogy), and I'm looking at a 24V system with Fogstar Drift lithium cells to keep costs down vs. Victron batteries.

Does the 3-day autonomy rule make sense, or should I be planning for more given UK winters? I keep seeing people say "oversize your battery" but at current lithium prices I'm trying not to go mental. Would a 200Ah 24V bank (roughly 4.8kWh usable at 80% DoD) be enough, or is that cutting it too fine?

@OddJobBob22 your post got cut off but I'll take a stab at it anyway.

The winter bit is what'll bite you. Solar input drops to roughly 20-30% of summer output in the UK, so you can't just size for your load — you're sizing for 3-4 consecutive grey days with minimal recharge.

Quick starting point:

1. List everything with watts × daily hours
2. Multiply total Wh by 4-5 days autonomy
3. Divide by 0.8 (don't drain lithium below 20%)

So if you're running 1kWh/day, you want ~5kWh usable minimum for winter.

Fogstar Drift cells are worth a look for DIY if budget's tight. Victron if you want it to just work without faffing.

What loads are you actually running? That's the number that matters before anything else.

@OddJobBob22 your post cut off so we're all guessing a bit, but I'd say don't panic — most people do overcomplicate this initially.

The practical starting point is to list every load with its wattage and realistic daily hours of use. Don't forget the sneaky ones like phone chargers, LED drivers, and any 12V fridge (which'll likely be your biggest ongoing draw).

For winter weekend use, I'd plan around 2-3 days of autonomy minimum given how grim UK solar can be November through February. That's not overcomplicating it, that's just sensible.

@GoldenNomad is right that winter will catch you out. What I'd add is: whatever figure you land on, factor in only using 50% of your usable capacity if you're going lead-acid, or around 80-90% for LiFePO4.

Post your full load list when you get chance and we can properly crunch the numbers with you.