Sizing a battery bank for a cabin used only a few weekends a month — overkill or worth it?

Looking at putting together a small off-grid setup for a weekend cabin in the Scottish Borders. The place gets used maybe 2-3 weekends a month, so the system sits idle most of the time. I'm torn between going with a modest 100Ah LiFePO4 and accepting some compromise, or going the full 200Ah route knowing it'll mostly sit at storage charge doing nothing.

For context, my boat runs a 200Ah Fogstar Drift bank with a couple of 175W panels and it works brilliantly — but that sees near-daily use in season. The cabin's a different beast entirely. We're talking LED lighting, a 12V compressor fridge, phone charging, and maybe a small 12V TV in the evenings. Total daily draw probably around 40-50Ah on a busy day.

The concern I keep coming back to is cycle life vs calendar life on the cells. If the battery's sitting at 50-80% SOC for three weeks at a time between visits, does the LiFePO4 chemistry handle that gracefully? Or am I better off with a smaller bank that gets more meaningfully cycled each visit?

Has anyone got a cabin setup in a similar pattern — infrequent use, long idle periods, Scottish or northern weather factored in? Curious what size you landed on and whether you'd do it differently now.

@SimonThompson the idle time is actually the crux of the issue here. LiFePO4 holds charge far better than lead-acid during those long weekday sit periods — self-discharge on a decent Fogstar or EVE cell bank is negligible over weeks. Lead-acid left partially discharged between visits will sulphate badly.

For a weekend-only cabin I'd actually argue bigger is better within reason. You want enough capacity that arriving Friday evening with a depleted bank from standby loads (alarm, router, etc.) doesn't mean rationing power all weekend.

My static van setup runs a 200Ah LiFePO4 with a Victron SmartSolar MPPT — the Bluetooth monitoring means I can check state of charge before I even leave home, so no nasty surprises on arrival.

What are your actual loads? Lighting, fridge, hot water? That'll define everything else.

Great points from @BatteryTony. One thing worth factoring in specifically for the Scottish Borders — your solar harvest in winter months will be quite poor, sometimes only 1-2 peak sun hours on a decent day. If the cabin sits empty for 10-12 days between visits, you need enough capacity that you're not arriving to a flat bank after a run of grey weather. I'd rather have slightly more capacity than you think you need than spend a Friday evening unable to run the kettle. Also worth considering a small trickle input from a wind micro-turbine as a supplement — the Borders gets reasonable wind resource year-round which helps compensate when solar undershoots. What loads are you actually planning to run? Heating, lighting, a fridge? That'll determine whether we're talking 5kWh or something beefier.

Really good thread this. @OffGridAlan makes a fair point about the Borders — I'd add that winter months up there you might be looking at genuinely poor solar days back to back, so whatever capacity you land on, factor in at least 3-4 days autonomy without meaningful generation.

One thing nobody's mentioned yet: a small trickle from a wind turbine could complement solar nicely in that part of Scotland, especially autumn through winter when the wind is far more reliable than the sun. Even a modest 400W unit keeps the bank topped up during the idle weeks.

On sizing generally — for a weekend cabin I'd lean towards slightly oversizing rather than under. The cost difference between "enough" and "comfortable headroom" is often less than people expect, and you'll thank yourself on a dark November Friday when you arrive and everything just works. 🏴󠁧󠁢󠁳󠁣󠁴󠁿

Something nobody's mentioned yet — if you ever want to charge an EV at the cabin (even just a slow overnight top-up via a 3-pin), your battery sizing calculation changes dramatically. Even a modest 10A overnight draw for 8 hours is 2.4kWh gone before you've boiled a kettle.

Worth deciding upfront whether EV charging is on the cards, because retrofitting extra capacity later costs more than spec'ing it right first time. I learned that the hard way.

On the idle time question — @BatteryTony is right about LiFePO4 self-discharge being negligible, but also consider a Victron BMS with remote monitoring. Being able to glance at state of charge from home before you head up for the weekend saves nasty surprises on arrival.

@MarshChild beat me to the EV bit but I'll add — running even a granny charger overnight absolutely demolished my assumptions about "small" battery banks on my shepherds hut build, ended up tripling my Fogstar capacity sharpish.

Good points all round. One thing worth flagging for a weekend-only place — battery health during the long idle periods matters more than people realise. Lead-acid will sulphate if left part-charged between visits; lithium (LiFePO4) is far more forgiving sitting at 50-80% for weeks at a time.

For the Scottish Borders specifically, I'd size your solar more generously than the battery — you can't always predict when you'll next visit to top things up manually. My garden office setup taught me that lesson the hard way one February.

Rough starting point:

• Battery: 200-300Ah LiFePO4 (Fogstar Drift are decent value)
• Solar: lean towards 400-600W given the latitude
• MPPT: Victron SmartSolar so you can monitor state of charge remotely between trips

That remote monitoring piece is underrated for a cabin — you can see if something's draining it before you arrive.

@CE_Builds raises something I've been caught out by with my shepherds hut setup. Left mine sitting at around 30% over a long winter and the Fogstar lithiums were fine but I'd been paranoid the whole time.

What's your solar input looking like @SimonThompson? Even a small 100-200W panel just ticking over keeps the bank topped up between visits and honestly that alone changes the whole idle period equation. Scottish Borders you'll get something even in winter.

Don't massively oversize the bank just for weekend use — the cycle count argument cuts both ways when cells are barely being used.