Battery bank sizing - how much capacity do I actually need for winter?

Right, so I've got a static caravan that's become a bit of a battery money pit and I need to work out if I'm being a complete muppet with my sizing.

Currently running a 10kWh lithium setup (Fogstar) which seemed sensible in summer when I'm barely using anything, but come November I'm hemorrhaging charge like the thing's got a puncture. Garden office runs off it too, plus I've got an EV to trickle-charge on sunny days (which, let's be honest, is about 90 minutes per month round here).

The actual question: I'm guessing winter means I should assume maybe 3-4 hours of usable solar per day maximum? My loads are roughly:

• Caravan essentials (heating, fridge, lights): ~2kWh/day
• Garden office (this is the culprit): ~4-5kWh/day
• EV charging when I can: wildly variable

So that's basically 6-7kWh minimum before I even look at the EV. Do I genuinely need to double my battery bank just to survive December, or should I be looking at a backup generator instead?

I've seen people mention the whole "80% usable capacity" rule and I'm wondering if I'm actually sizing for 8kWh when I should be looking at 20kWh, which sounds like financial suicide.

What's everyone's approach? Are winter setups just inherently expensive, or have I made some catastrophic design error?

Mate, 10kWh is actually decent for a static setup—problem is most people underestimate winter losses. You're not being a muppet, you're just hitting reality.

The real question is: what's your actual daily consumption in December/January? Solar production drops to about 30-40% of summer figures up here, so if you're pulling 5kWh daily in winter, you need enough capacity to bridge those grey days and maintain usable depth.

With Fogstar kit, you've got good BMS integration with Victron, which helps. What's your solar array size? That's the bottleneck—I run 6kWh LiFePO4 with 4.5kW panels and still struggle through February without grid backup. Static caravan advantage is you can actually add panels without weight penalties.

Honestly, unless you're running immersion heating or similar, 10kWh should handle most scenarios. What's actually draining the money—replacements, or just the upfront cost?

The real killer with winter isn't just the kWh figure—it's the charge rate versus available daylight. You could have 20kWh sat there doing nothing if your solar can't replenish it fast enough between November and February.

What's your actual solar capacity and panel angle? Static setups have a massive advantage over boats because you can optimize orientation, but most folks bolt panels on and hope. I'd be looking at whether you're genuinely running short or just watching the percentage drop slowly over grey weeks and panicking.

10kWh's reasonable if you're running modest loads (heating excluded—that's a separate conversation). The Fogstar gear's solid but the real test is: are you actually depleting to critical levels, or just watching it sit at 40% for days?

Post your average daily winter consumption and solar specs and we can actually work out if you need more capacity or better charge sources.

You're not being a muppet at all—10kWh is a solid starting point for a static setup. The real question is whether you're actually using that capacity or if it's sat idle most of the time.

What @Ozzy and @ExSquaddie49 are getting at is spot on. Winter's brutal because you're dealing with three simultaneous problems: reduced solar output, lower charging efficiency in cold temps, and longer nights. Your Fogstar should handle the depth-of-discharge fine, but I'd focus on your actual consumption patterns first.

Log your daily usage for a fortnight in January—peak winter. Are you hitting 5kWh daily? If you're consistently under 6-7kWh even with heating running, you're actually oversized (which isn't the worst problem). If you're regularly draining past 80%, then you might genuinely need to look at either additional capacity or beefing up your charge sources.

What's your solar setup like currently?

Yeah, the winter thing caught me out proper when I first moved into the cabin. 10kWh sounds like loads until December rolls round and you're sat there watching the charge percentage drop faster than it climbs.

Real talk though—static caravans are different beasts from vans because you're not moving, so you've got space for proper solar coverage. That's your ace card. I've got about 6kW of panels scattered across the roof and it makes a massive difference even on grim days.

The muppet question isn't really about the battery size, it's whether your charging sources match the winter draw. @ExSquaddie49's bang on about daylight hours being the actual bottleneck. If you're running resistive heating alongside typical loads, 10kWh can vanish overnight.

What's your panel wattage looking like? That'll tell you whether you need more battery or just better charge capability.

@BorderVanLifer - Cheers for sharing the actual figures. 10kWh is decent, but here's the honest bit: winter capacity needs depend massively on your actual usage patterns rather than some textbook calculation.

Have a look at your consumption logs from December/January if you've got them. Are you running heating? That's the real battery killer. If it's just lights, fridge, and modest loads, 10kWh should get you through most days even with poor solar input.

The other factor @ExSquaddie49 mentioned is spot on—you could have unlimited capacity but if you're only getting 2-3 hours decent charging time in winter, you're still fighting a losing battle without either a backup generator or reducing consumption.

What's your typical daily kWh draw on a winter's day? That'll tell us whether you need more storage or just a different approach to managing winter.