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

Running a similar sizing exercise for my static caravan setup last year, so genuinely curious what others think here too.

My situation: small space, modest loads — LED lighting, a 12V compressor fridge (roughly 45Ah/day in summer), phone charging, and occasionally a 12V fan. Running the numbers I land somewhere around 60-80Ah of usable capacity needed per day, assuming one overcast day of buffer.

With lithium you're working with ~80% usable capacity realistically (though Fogstar and most reputable cells will handle deeper than that). So a 100Ah lithium arguably covers my use case with headroom. The 200Ah feels like it'd be sitting at 70-80% state of charge most of the time, which isn't necessarily bad for longevity, but you're paying for capacity you're not cycling.

The counterargument I keep coming back to though — shepherd's huts often have limited roof space for solar, so if your charge input is restricted (say 200W panel max), a larger battery gives you more runway through poor weather without the bank bottoming out.

What's your actual solar array size? And are you grid-tied at all for backup, or fully off-grid? That'll change the calculus significantly.

Also worth knowing: are you on 12V or 24V? If the hut might expand in future (more loads, a small inverter for a laptop etc.), sizing up now on a Victron or Fogstar unit could save a retrofit headache later.

@Cerbo_Geek depends entirely on your solar input and how many cloudy days you need to ride out. For a shepherd's hut with those loads I'd say 200Ah is probably fine rather than overkill — lithium's usable capacity is roughly double equivalent lead-acid, so you're not swimming in headroom.

I've got a similar modest setup in my garden office running on a 100Ah Fogstar Drift and honestly that covers everything comfortably with a couple of decent panels. The compressor fridge is the killer load to size around — what's its rated draw?

One thing worth considering: lithium doesn't sulphate sitting partially charged the way lead-acid does, so if the hut sits empty for weeks at a time that 200Ah will hold its state of charge far better than a smaller lead-acid bank would.

@Cerbo_Geek 200Ah lithium is only "overkill" until your first proper British winter week when the sun disappears for six days straight and you're rationing phone charge like it's wartime.

Hey @Cerbo_Geek, great question. Worth thinking about usable capacity too — with lithium you're realistically pulling 80-90% of that 200Ah, so closer to 170Ah in practice. For a shepherd's hut with LED lighting and a 12V compressor fridge, I'd actually say 200Ah is probably the minimum I'd be comfortable with rather than overkill. Compressor fridges are brilliant but they cycle constantly and chew through more than people expect, especially overnight when there's no solar input. @BoatEwan and @MultiPlusFan both make solid points about the winter factor — up here in Cumbria I've seen three or four genuinely sunless days back to back in November. You'll thank yourself for the headroom. What's your planned solar array size? That'll change the answer considerably.

@CumbrianExplorer makes a solid point on usable capacity, but nobody's mentioned the charging side yet.

A 200Ah LiFePO4 wants a reasonably sized solar array to actually fill it — at minimum 300W realistically, more in a hut context where panels are often compromised by roof angle or partial shading. I run a Fogstar 200Ah in my own shepherd's hut paired with 400W and even that feels tight October through February.

Also worth considering: if you're planning any EV charging (even a trickle via a 13A outlet), 200Ah will feel very small very quickly.

Run your actual loads through something like Victron's MPPT calculator before deciding — "small" loads have a habit of not being as small as assumed once you tot everything up properly.

@SomersetVanLifer raises something I learned the hard way on my hut build — the charging side really does dictate everything.

What's your solar panel wattage looking like, @Cerbo_Geek? A 200Ah Fogstar Drift sitting behind a single 100W panel is going to be chronically undercharged through October–March regardless of your load profile.

For a shepherd's hut specifically, I'd actually argue 200Ah is about right rather than overkill — the thermal mass of those small spaces means you might lean on a 12V fan heater or heated blanket more than you'd expect. My hut genuinely surprised me with overnight draw once autumn hit.

If budget's tight, a single 100Ah to start isn't terrible, but buy a battery monitor from day one so you can see exactly what you're actually using before committing to more capacity.