So I'm about three months into building a small off-grid cabin in mid-Wales, roughly 20 square metres. It's purely a weekend and holiday retreat, so realistically it'll only be occupied maybe 10-15 days a month at most, probably less in winter. I've got two 200W panels on the roof feeding into a Victron SmartSolar 100/30 MPPT, which feels like plenty for what I need.
The confusion I keep running into is battery sizing. I've seen the usual advice about 3-5 days of autonomy, but that seems almost absurd when the cabin sits empty for weeks at a time. My actual loads are pretty modest — a 12V LED lighting circuit, a small 12V compressor fridge (around 30-40Ah per day), phone/laptop charging, and occasionally a 12V water pump. I'm estimating maybe 60-70Ah per day of actual use when someone's there.
I've been going back and forth between a single 100Ah LiFePO4 and going up to 200Ah. The 100Ah would cover a couple of cloudy days no bother, and with two panels even a dull Welsh winter day should knock out a decent chunk of charge. But I wonder if I'm being naive about how bad it can actually get up there in January and February — we're talking proper overcast for days on end sometimes.
Has anyone sized a system for genuinely intermittent use like this rather than permanent living? Does the "days of autonomy" rule even make sense when the cabin's empty half the month and you've got panels topping it up continuously anyway?