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

Got a similar dilemma at the moment so following this thread closely.

My setup is purely for emergency backup — mainly keeping a few lights going, charging phones, and running a small 12V fan if needed. Nothing exotic.

I've been looking at the Fogstar Drift 200Ah because the price has come down quite a bit recently, but I genuinely can't work out if it's massive overkill for what I need or whether the extra headroom is actually worth having long-term.

A few questions I keep coming back to:

• Does regularly running a lithium battery at, say, 10-20% of its capacity cause any issues with cycle life?
• Would a 100Ah be perfectly adequate for occasional weekend use, or is bigger always better with lithium given the partial state-of-charge tolerance?
• Is there any real-world downside to oversizing beyond the obvious cost and weight?

The hut itself is pretty small — one room, no mains hookup, and it won't be lived in full-time. Probably used maybe 2-3 weekends a month at most.

I've had a look at a few Victron battery monitors and I'm wondering if the smarter move is just getting a decent 100Ah with proper monitoring rather than throwing capacity at the problem blindly.

Has anyone actually sized a battery for a similar low-demand, intermittent-use situation? Would love to know what you landed on and whether you regret going bigger or smaller.

HighlandDweller | Posts: 847

@WheresMeWires31 For that kind of light-duty emergency backup use, a 200Ah lithium is almost certainly overkill, aye. You'd realistically be looking at maybe 20-30Ah daily consumption for lights, phone charging, and a small fan — so even a 100Ah battery gives you three days' comfortable buffer without any recharging.

Worth considering though: lithium's flat discharge curve means you actually use what's rated, unlike lead-acid where you're working with maybe 50% usable capacity. So the comparison isn't always straightforward.

If budget's tight, a quality 100Ah LiFePO4 would serve your emergency needs perfectly well and leave money spare for decent solar panels — which honestly matter more in the long run, especially up here where we don't exactly get the Sahara's sunshine! 😄

WattAMess25 | Posts: 312

@WheresMeWires31 Worth sharing my own journey here — I started with a 100Ah Fogstar Drift on the boat thinking it was plenty, then slowly added bits and pieces until I was scratching my head wondering why it felt tight.

The honest question isn't really "is 200Ah overkill?" — it's how many consecutive cloudy days do you want to ride out without anxiety?

For your load profile specifically, I'd roughly pencil in:

• Lights: ~10–20Ah/day
• Phone charging: ~5Ah
• Fan: ~20–30Ah depending on hours

That's maybe 50Ah worst case daily. A 100Ah lithium (usable ~95%) covers two days comfortably. A 200Ah gives you breathing room without the mental arithmetic.

Only you know if that peace of mind is worth the extra outlay.

Rodney | Posts: 203

@WheresMeWires31 Worth doing the maths first — add up your daily Wh use before buying anything. Lights, fan, phone charging... bet you're under 20Ah/day easy.

For pure emergency backup I'd actually look at something smaller like a 50-100Ah rather than 200Ah. Less to charge, less weight, cheaper. Fogstar do decent smaller cells that won't break the bank.

The one thing I'd say is — what's your charging source? If you're relying on solar only, UK winters can be brutal. My garden office setup taught me that lesson the hard way. Ended up undersized on panels, not the battery.

CaddyProject | Posts: 1,156

@Rodney is right — do the load calc first. For what you've described, rough ballpark:

• LED lights (say 2 × 5W, 4hrs) = 40Wh
• Phone charging (2 phones) = ~20Wh
• 12V fan (20W, 3hrs) = 60Wh

That's ~120Wh/day. Even a 50Ah battery gives you 600Wh usable (lithium, 100% DoD). That's 4+ days without any input.

100Ah is probably the sweet spot for your use case — something like the Fogstar Drift 100Ah would do it comfortably. 200Ah isn't "overkill" if you ever want to expand, but for purely emergency backup you're paying for capacity you'll rarely touch.