Anyone know if a 200Ah lithium battery is overkill for a small shepherd's hut?

Been mulling this over for a few weeks and could do with some input from people who actually know what they're talking about.

I'm helping a mate set up a small shepherd's hut — roughly 4x2 metres, used mainly for weekend stays. The loads are pretty modest: a few LED lights, phone charging, a small 12V fan, and occasionally a laptop. No cooking off the battery, she's got a wood burner for heat.

I've got a spare 200Ah lithium (Fogstar Drift, 12V) sitting in my narrowboat workshop that I could let her have at a decent price. But I'm wondering if it's genuinely useful capacity or just unnecessary weight and cost for what she actually needs.

My gut says even 100Ah would be plenty for this use case — maybe even 80Ah if the solar input is decent. The hut will have a couple of panels on the roof (probably 2x 175W Renogy), so recharge shouldn't be an issue in decent weather.

A few specific questions:

• Is there a minimum usable size where lithium stops making sense vs AGM?
• Would a smaller battery actually cycle more aggressively and wear faster with modest loads?
• Any reason to keep the 200Ah rather than selling it and buying something smaller purpose-built?

I've done plenty of this on my own boat setup (Victron kit throughout) but a shepherd's hut feels like different territory — more seasonal, sitting unused for longer stretches. That idle time factor is what's making me second-guess the whole thing.

Anyone run a similar small setup? What did you land on?

@BoatPete mate, "overkill" is just another word for "sorted" — stick the 200Ah Fogstar in and stop worrying about it.

@BoatPete the "overkill" question is meaningless without knowing the actual loads. What's going in — 12v lighting, a small inverter, phone charging? Or are we talking a proper off-grid setup with a fridge and maybe EV top-ups?

I've got a boat setup and oversized my bank initially — no regrets. But I'd want to know:

• Daily consumption estimate (Wh)
• Solar input (how much panel, what orientation)
• How many cloudy days do you need to ride out?

A 200Ah LiFePO4 gives you roughly 200Ah usable (unlike lead-acid where you're stuck at 50%). For a shepherd's hut with modest loads and decent solar, it's probably comfortable rather than overkill. For a weekend-only setup with minimal loads, you might get away with 100Ah — but you'd feel it in a bad week of weather.

@ExBrickie is on the right track — loads first, battery second.

That said, for a shepherd's hut I'd broadly say 200Ah is fine not overkill. Small spaces tend to creep up on you: lighting seems trivial until you add a phone charger, a small fan, maybe a 12v coolbox. Suddenly you're pulling 30-40Ah a night without thinking about it.

I've got a garden office setup on Fogstar 200Ah and honestly I wouldn't go smaller. The real question is your solar panel sizing — undersized panels with a big battery just means a flat battery by day three of grey skies, which is very much a UK problem.

What's the panel situation? If it's only 100W on the roof, you'll want to be conservative regardless of battery capacity.

@BoatPete worth thinking about how often it'll be used as well as what's running off it. My narrowboat setup taught me that a battery sitting largely unused kills itself faster than one that's being cycled regularly — lithium especially wants to be kept between 20–80% and actually used.

If your mate is only popping in at weekends, a 200Ah Fogstar might sit at 100% for days on end. Has he thought about what's doing the charging — solar panel on the roof, hook-up, or both? Because that changes the equation quite a bit.

Also curious — is there any chance EV charging ends up on the list later? Even a small top-up charge for an ebike would swing the load calculations noticeably.