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

Been lurking on this thread with interest because I'm asking myself something very similar about my static caravan setup.

I've got a little off-grid retreat in the Scottish Borders — no mains connection, just a modest solar array and currently an ageing lead-acid battery that's given up the ghost over this past winter.

Before I pull the trigger on a 200Ah Fogstar Drift lithium (which everybody seems to rave about on here), I want to make sure I'm not massively oversizing things. The caravan runs:

• LED lighting throughout
• A 12V compressor fridge
• Phone and tablet charging
• Occasionally a small fan in summer

I've calculated roughly 60-70Ah actual daily usage, which to my nurse's brain means 200Ah lithium should theoretically give me comfortable headroom — especially since you can actually use 80-90% of lithium capacity rather than the 50% rule with lead-acid.

But here's what's nagging at me: is there any disadvantage to having more capacity than you strictly need? I keep reading that undersizing your solar relative to battery size causes problems with not reaching full charge regularly. My current panels are only 2×100W Renogy, which feels a bit lean for a 200Ah bank now that I think about it.

Would a 100Ah lithium actually serve me better and be easier to keep properly charged with my existing panels? Or is bigger genuinely better when you factor in a few cloudy Scottish days in a row?

Anyone with a similar modest setup gone down this road and lived to tell the tale? I'm all for not reinventing the wheel when this community clearly has so much collective experience.

@RetiredNurse58 Scottish Borders with no mains = you'll want every Ah you can get through those grey winter months, honestly 200Ah lithium (grab a Fogstar Drift if budget's tight) paired with a halfway decent Victron SmartSolar MPPT is practically the minimum I'd suggest for anything beyond a single lightbulb and phone charger — the real question is your panel wattage, because a beefy battery with stingy solar is just an expensive paperweight waiting to go flat by Tuesday. ☀️🏴󠁧󠁢󠁳󠁣󠁴󠁿

@RetiredNurse58 200Ah isn't overkill at all for a static caravan — it's probably the minimum I'd suggest. Bear in mind you're only realistically using 50-80% of usable capacity on LiFePO4 before the BMS starts getting grumpy, so 200Ah is effectively ~160Ah in practice.

Scottish Borders winters are grim for solar too. I'd actually be looking at:

• 200Ah as a starting point, not a ceiling
• A decent MPPT controller (Victron SmartSolar handles low-light surprisingly well)
• Backup charging plan — small generator or wind if you're exposed enough

What's your typical daily load like? Lighting, fridge, phone charging? That changes everything. Fogstar do decent value 200Ah cells if budget is tight.

@BoatPaddy is right about the grey months — I've had days in December where my panels barely contributed anything useful.

@RetiredNurse58 Worth doing a proper load calculation before committing to a size. Roughly — add up your daily watt-hours (lights, fridge, phone charging etc.), divide by your battery voltage, and that gives you your daily Ah draw. Then factor in that with lithium you realistically want to stay above 20% SOC, so your usable capacity is 80% of the rated figure.

For a static unit with a fridge running continuously, I'd also ask: what's your solar input like up there in winter? Scotland's irradiance figures are noticeably lower than southern England — a 200Ah bank with insufficient charging is just an expensive problem waiting to happen. Are you pairing it with solar, or thinking about a backup charger as well?

@BevJackson64 has the right idea but nobody ever does the load calc honestly — they forget the kettle, the phone charger, the "I'll just quickly hoover" moment, and suddenly 200Ah is gone by Tuesday.

Scottish Borders winters are brutal on solar yield too; I run a 200Ah Fogstar Drift on my narrowboat and even I get nervous in January when we've had three days of solid grey.

Size up, not down — a properly managed lithium at 50% DoD lasts far longer than one you're hammering to the knees every cycle.