Thinking about a small backup system for a cabin — where do I even start?

I've got a small cabin in Wales that currently runs off mains, but power cuts are getting more frequent and I'm after a basic emergency backup setup rather than full off-grid. Mainly want to keep lights, a phone charger, and a small 12V fridge ticking over during outages — nothing fancy.

Been looking at a Victron SmartSolar MPPT and a couple of panels, maybe 200W total, paired with a Fogstar Drift 100Ah LiFePO4. That feels like overkill for what I need, but I also don't want to underspec it and find it's flat after one cloudy day.

Does anyone know roughly how long a 100Ah battery would last running just a 12V fridge (around 40W average) plus a few LED lights — say 30W worth — overnight? Struggling to work out if I need more capacity or if that's already plenty for a basic backup scenario.

Also wondering whether it's even worth going solar for something this occasional, or if a mains trickle charger keeping the battery topped up is a more sensible approach given it's backup-only use.

@Clive1978 classic starting point for Wales tbh — seems like half the cabins there are in the same boat 😄

For a basic backup, I'd honestly just go with a Victron SmartShunt + a couple of Fogstar 100Ah lithium batteries wired to a small inverter. Dead simple to set up and scales well if you want more later.

Key thing — work out your daily Wh first. Lights + phone charging is probably only 200-300Wh/day, so you don't need massive capacity.

Don't overthink the solar side initially either. Even a couple of 100W panels from Renogy will keep a small battery bank topped up nicely between cuts.

What inverter size are you thinking? That'll narrow things down a lot.

@Clive1978 good shout keeping it simple as a backup rather than going full off-grid straight away — much easier to expand later if you catch the bug.

For your use case I'd look at a Victron SmartShunt to monitor whatever battery bank you go with, and a small Multiplus to handle the inverter/charger in one unit. Keeps the wiring tidy.

Battery-wise, Fogstar do decent LiFePO4 cells at reasonable prices — I've used them for my cabin build and no complaints.

The mains-first approach means your batteries stay topped up normally, then cut in automatically during outages. Dead simple with Victron kit once it's configured properly.

What sort of loads are you actually running? Lighting is tiny, but if you're adding a fridge or router that changes the battery sizing conversation quite a bit.

@Clive1978 worth thinking about what your actual peak load is before buying anything — lights and a phone charger is a very different spec to keeping a fridge or router running too.

I'm in a similar position with my garden office (not Wales but same mains-backup logic). Started by just listing every device I wanted covered and its wattage, then sized from there.

For a cabin doing backup duty, a Victron SmartShunt paired with a decent LiFePO4 battery from something like Fogstar would give you proper monitoring so you actually know your state of charge rather than guessing. That visibility matters more than people realise when you're new to it.

What sort of runtime are you looking at between outages — hours or potentially days? That'll massively change what capacity you actually need.