What's the minimum viable battery capacity for a shepherd's hut emergency backup?

Been mulling this over for my hut setup. Currently running a 200Ah Fogstar Drift LiFePO4 and 400W of Renogy panels, but trying to work out what the actual floor is for a usable backup — not "keep the lights on" but genuinely functional (fridge, lights, phone/laptop charging, a fan or small heater).

My rough numbers suggest a fridge pulls ~40-50Ah/day, lighting another 10Ah, and misc USB/laptop maybe 15Ah. So call it 65-75Ah daily minimum. With a Victron SmartShunt keeping tabs, I can see exactly where it's going — and honestly the fridge alone is the killer.

So the question is: what are others running for backup specifically — i.e. the reserve you'd never normally touch? I keep 50% SOC as my floor given LiFePO4 cycle life, which on a 200Ah bank means 100Ah usable reserve. Feels tight if solar's out for 2-3 days (UK winter, obviously).

Is 200Ah the sensible minimum or would you go bigger before adding more panels?

DaiCole | 847 posts

Good question @CurrentAffairs. Honestly for a shepherd's hut I'd argue 100Ah LiFePO4 is the realistic floor, assuming you're not running anything daft like an electric shower.

That gets you:

• LED lighting through the night
• Phone/device charging
• A 12V fan or small water pump
• Maybe a 12V compressor fridge on low duty cycle

The fridge is really the swing factor — drop that and you could argue 50Ah covers genuine emergency use for a few days.

Worth being honest about what "emergency" actually means to you though. Cold dark nights with hot food from a gas hob? Very different to wanting normal creature comforts when the panels aren't producing.

Your 200Ah setup sounds well sorted for day-to-day. What's prompting the rethink — planning a second, leaner unit somewhere?

ExPostie86 | 312 posts

@CurrentAffairs your 200Ah Fogstar is already doing the heavy lifting — the real "floor" question is what specifically needs keeping alive during an outage, because "emergency backup" in a shepherd's hut with LED lighting and a phone charger is a very different animal to one running an inverter and a milk frother.

From my narrowboat days: I'd say 50Ah usable (so ~100Ah LiFePO4 at 50% DoD if you're being cautious, though Fogstar cells handle deeper cycles fine) covers lighting, USB charging, and maybe a 12V water pump for 24–48hrs.

Anything with resistive heating blows that maths out the window immediately though — that's where people get caught out.