Struggling to size my battery bank for a winter cabin — am I overthinking this?

Finally pulling the trigger on a proper off-grid setup for my little timber cabin in the Peak District. It's not a full-time residence — mostly weekends and the odd week away — but I want it to be genuinely comfortable, not just "camping with walls." I've been running numbers all week and I keep second-guessing myself.

Current load estimate is roughly 2.8–3.2 kWh per day in winter: LED lighting, a 12V compressor fridge, laptop, phone charging, and a small inverter for the odd power tool. I'm planning 400W of solar (two Renogy 200W panels) but obviously winter in Derbyshire means I might get 1–2 peak sun hours on a bad week. I was thinking 2× Fogstar Drift 200Ah 12V LiFePO4 in parallel (so 400Ah / ~4.8 kWh usable at 80% DoD), with a Victron SmartSolar MPPT 100/30 to tie it together.

The maths says I'd have roughly 1.5 days of autonomy without any solar input, which honestly feels a bit thin for a cabin I might leave unattended in January. Has anyone sized for genuine UK winter use and found that's actually fine, or did you wish you'd gone bigger? Also wondering if a small petrol/propane generator as a backup makes more sense than throwing another £400 at a third battery.

@ExpertCamper Winter in the Peak District is no joke — I'd factor in 3-4 days of autonomy minimum given how grey it gets up there December through February.

A few things worth nailing down first:

• Daily Wh consumption (be honest, add 20% buffer)
• Usable capacity — don't go below 20% SOC on lithium, 50% on lead-acid
• Worst-case solar input — I'm talking 1-2 peak sun hours in deep winter

I run a similar setup on my boat for emergency backup purposes and Victron's BatteryProtect combined with Fogstar Drift lithium cells has been rock solid. Lithium is genuinely worth the premium for a cabin that'll sit unused for weeks — self-discharge is minimal and you won't babysit it.

What's your rough load list looking like? That'll tell us whether you're genuinely oversizing or not.

HeatherMandy | 847 posts | ⭐ Trusted Member

@ExpertCamper Don't worry, we've all been there with the sizing paralysis! One thing I'd add to @SimonThompson's autonomy advice — really nail down your actual loads first before worrying about battery capacity. People consistently overestimate what they need. Make a proper spreadsheet: every appliance, wattage, and realistic hours of use per day. Winter lighting and phone charging are trivial; it's heating controls, a laptop, or worse — an electric kettle — that'll surprise you.

Also worth noting that usable capacity varies significantly between battery chemistries. LiFePO4 will give you roughly 80-90% usable capacity whereas lead-acid you're looking at 50% max if you want decent longevity.

What inverter size are you considering, and are you planning solar only or adding a small wind turbine? The Peak District wind resource is actually decent! 🌬️

@ExpertCamper I had almost identical headaches sizing up my static caravan setup before I landed on something sensible. The thing that finally clicked for me was separating critical loads from comfort loads. Write them down in two columns. Critical stuff — fridge, lighting, phone charging — runs no matter what. The kettle and telly? Luxuries when the sun's generous.

Once I'd done that, the maths practically sized itself. I ended up with a 200Ah Fogstar Drift LiFePO4 for the caravan, and honestly the 3-day autonomy @SimonThompson mentions is spot on for grim British winters. Peak District in January makes my place look positively Mediterranean.

Also worth noting — LiFePO4 gives you proper usable capacity rather than that lead-acid 50% nonsense. Factor that in and your bank suddenly looks much healthier on paper.

@ExpertCamper Worth thinking about what your actual worst-case daily consumption looks like rather than just the total bank size. I made the mistake with my boat of sizing for "average" days and got caught out badly in November.

What loads are you running? Lighting, heating controls, a fridge? The heating side catches people out — even a small 12v pump for a log boiler can add up overnight.

@SimonThompson's point about autonomy days is solid. On my tiny house setup I went with Fogstar Drift LiFePO4 cells — the price-per-kWh made sense for a part-time setup where I didn't want to overbuy on expensive Victron Smart Lithium.

Have you done a rough load calculation yet, or are you still at the "what do I even need to power" stage?