Best way to size a battery bank for a small cottage with occasional use?

Right, so I'm trying to work out battery sizing for a small cottage I've just picked up. It's only used weekends and the odd week during summer — nothing crazy.

Currently looking at running basics: lighting, a small fridge, phone charging, maybe a laptop. No heating or hot water from the system (that's got its own gas setup).

I've got a rough solar array planned — couple of 400W panels — but want to make sure the battery bank doesn't end up being overkill or undersized. With the irregular usage pattern, I'm wondering if I should be sizing based on worst-case (dead of winter, cloudy week) or something more realistic given we're mainly there when it's decent weather.

Been reading about the Victron setup some folks use, and there's loads of different approaches floating about. Some people seem to go massive with their banks and I'm not sure if that's necessary or just peace of mind.

Also, should I factor in future expansion? Might want to add an EV charger or proper heating down the line, but not immediately.

Any rules of thumb for occasional-use properties? Is it really just daily load × days of autonomy, or does the occasional-use bit change the maths?

Cheers for any pointers — trying to get the spec right before ordering parts.

Cottage weekends are actually ideal for batteries because your usage pattern is predictable. The trick is sizing for your peak day consumption, not average.

For occasional use, I'd suggest working backwards from what you actually need:

Map a typical weekend: kettle, lighting, fridge, maybe a shower pump. Jot down wattages and hours. Most cottages realistically hit 5-10kWh per day.

Add a safety margin (25%), then size your battery to handle that plus cloudy days. A Victron MPPT controller with monitoring lets you see real consumption data after install — invaluable for fine-tuning.

Since you're only there occasionally, a smaller bank (8-12kWh LiFePO₄) with decent solar usually outperforms a massive lead-acid setup gathering sulphate. The DoD flexibility of lithium works brilliantly for this pattern.

What's your typical power draw looking like? That'll nail the sizing.

The key thing is working backwards from what you actually use those weekends, not what you think you might need. Jot down every appliance you'll run and for how long — kettle, lights, fridge, whatever.

Then factor in that you're only there intermittently. Your battery can recharge fully between visits, which is brilliant. A lot of folk oversize because they're anxious, but that just costs more upfront and degrades faster.

I'd aim for 2-3 days autonomy max. With a modest solar array topping up during the week, you'll rarely need to touch it. Something like a Victron LiFePO₄ in the 5-10kWh range is probably the sweet spot for a cottage setup.

What're you thinking of powering? That'll help narrow it down.

Right, I'll tell you what I learned the hard way with my narrowboat setup—and cottages follow the same logic. The killer mistake is oversizing because you might need it. That's just expensive sitting around.

Start with an actual weekend: kettle, lights, fridge, laptop charging. Write it down. Seriously. Most people wildly overestimate.

Here's the thing though—cottages have a quirk motorhomes don't: if you're only there weekends, your battery can fully recover mid-week on solar or mains charging. That means you can run smaller than you'd think. A 5kWh Victron or Fogstar system handles most cottage weekends comfortably, which costs a fraction of what people typically buy.

And crucially: work backwards from your usage pattern like @PartnerAdventure said, but also factor in seasonal variation. Summer's easy; winter weekends are tighter. What sort of load are we talking—just essentials, or are you planning proper comfort?

You're in a good position with intermittent usage — the hard part is nailing your actual consumption figures. Have you measured what you're actually running those weekends? Not guesses, but actual wattages?

I'd suggest a week or two of logging everything: kettle, lighting, fridge, whatever. That'll give you real numbers to work backwards from.

One question though — are you planning mains backup, or pure off-grid? That changes the battery sizing completely. If you've got grid available as fallback, you can size smaller and let the battery handle daily cycling. Pure off-grid means you need more buffer for cloudy stretches.

What's your charging setup looking like? Solar, wind, or hybrid? That factors heavily into whether you're better with lithium or LiFePO₄ — the depth of discharge tolerance makes a big difference on smaller systems like yours.

@Davo83 — have you actually measured what you're pulling during those weekends? That's where most people go wrong. I've got a similar setup and the difference between what you think you'll use and what you actually do is massive.

Get a clamp meter and track a typical weekend — lights, fridge, heating, whatever. Then work backwards from there. For occasional use like yours, you might be looking at 10-20kWh depending on what appliances you're running.

One thing worth considering: with intermittent usage, your battery stays in better health if you're not cycling it heavily every week. Might be worth slightly oversizing rather than going minimal — gives you more flexibility and the battery degradation curve works in your favour.

What sort of load are we talking? Daily usage in winter vs summer makes a massive difference.

@Davo83 for intermittent use, oversizing the battery a bit actually pays off — batteries left at partial state of charge for weeks at a time take a hammering. Learnt this with my shepherd's hut setup.

I'd say whatever figure you land on, add 20-30% buffer. Also factor in whether you'll have solar topping it up between visits or if it's sitting idle — makes a big difference to what chemistry makes sense.

LiFePO4 is the obvious shout (Fogstar do decent value cells), but if budget's tight and the cottage is genuinely occasional use, a good AGM bank isn't a disaster.

What's your solar/charging situation looking like?

@TracyKnight makes a fair point about oversizing, and it reminded me of something I learned the hard way on my boat.

I sized exactly to my calculated needs, then discovered that "occasional use" often means arriving Friday night in January, cold and dark, hammering the system all at once rather than spreading load across a week.

Peak demand on arrival day will dwarf your average figures. I'd factor in at least one full "worst case" evening — every light on, kettle going, phone charging — and size from that, not your daily average.

My Fogstar cells have been solid for exactly this kind of irregular, bursty use.

@Davo83 here's the bit nobody mentions: weekend-use properties are brutal on batteries because you're constantly cycling them from full → partial → sitting for 5 days → repeat.

With LiFePO4 that sitting period is basically irrelevant — unlike lead-acid which sulphates when left low. So honestly, for intermittent cottage use, LiFePO4 isn't just a nice-to-have, it's the sensible choice.

My motorhome setup taught me this the expensive way — two AGM banks destroyed in 18 months before I switched.

Rough rule: calculate your daily consumption, multiply by 2 for autonomy, then don't derate for depth-of-discharge like you would with lead-acid. Fogstar Drift cells are worth a look for the budget.