Static caravan solar - worth going bigger on the battery bank from the start?

Finally got the 400W of Renogy panels up on the static last month. Running a Victron MPPT 100/30 into a pair of 100Ah Fogstar Drift LiFePO4s - so 200Ah total at 12V. Seemed plenty on paper.

Problem is we're up in Scotland so winter days are brutal. Getting maybe 1-2 usable hours of decent generation some days, and the bank is down to 40-50% by morning after running just the lights, a 12V fridge, and phone charging overnight. Nothing dramatic, but it's tighter than I'd like.

Starting to wonder if I should've just gone 400Ah from the off rather than planning to "expand later." The Fogstars are easy enough to add to in theory, but you're supposed to match batteries properly and mine are already 6 months into their cycle count.

Has anyone retrofitted extra capacity to an existing bank mid-life, or is the practical answer just to accept the loss and start fresh with a bigger matched set? Curious what others did with their cabins or shepherd's huts over winter specifically.

@BoatMick sounds like you've made a solid start there! I went through exactly this with my static a couple of years back and honestly, yes - go bigger from the start if you can. The cost per kWh drops significantly when you add batteries early rather than retrofitting later.

The thing nobody tells you is that 200Ah sounds generous until you hit a run of grey November days and you're nursing everything carefully. I'd say 400Ah minimum for comfortable year-round use in the UK, especially if you're running a fridge.

Your Victron MPPT is already well-specced so it'll handle the extra capacity no bother. Fogstar Drift cells are good quality too so worth expanding within the same bank.

What's your typical daily load looking like? That'll help work out whether 400Ah would genuinely sort you or if you'd be better considering 600Ah.

@BoatMick the core issue is that 400W of panels into 200Ah is actually a reasonable ratio during decent generation hours, but your usable capacity is only as good as your worst consecutive cloudy days — and in the UK that can easily be 4-5 days, especially November through February.

The MPPT 100/30 will happily handle a 400Ah bank if you expand later, so the controller isn't your bottleneck. Adding another pair of Fogstar Drifts in parallel is straightforward if your cabling's sized correctly from the outset.

One thing worth checking: what's your actual resting consumption overnight? EV charging ambitions aside, even a modest 3A parasitic draw will chew through 200Ah faster than people expect. Log it properly with a Victron BMV-712 before deciding whether it's genuinely a capacity problem or a consumption audit problem.

@BoatMick the shepherd's hut I run taught me this lesson the hard way — you don't notice the battery ceiling until the first run of grey November days hits back-to-back.

Your Fogstar Drifts are good cells, but 200Ah at 12V is only 2.4kWh usable assuming you're keeping above 20% as you should. That evaporates fast once you factor in an evening's lighting, a 12V fridge cycling overnight, and maybe a laptop or two.

What I'd genuinely suggest: price up a third Drift now rather than scrambling for it mid-winter. Fogstar's pricing makes incremental expansion relatively painless compared to some alternatives, and your MPPT 100/30 has headroom to handle the extra capacity without any controller changes.

The panel-to-battery ratio @QJ_Builds mentions is fine when the sun shows up — the battery bank is your insurance for when it doesn't.

Hey @BoatMick, worth mentioning one thing nobody's touched on yet — check what your actual overnight draw is before committing to more capacity. I made the mistake of just chucking more batteries at my setup before I'd properly audited my loads. Turned out a cheap inverter was pulling nearly 2A in standby all night, which was my real problem rather than insufficient storage. Grab a simple clamp meter or hook a Victron BMV shunt in if you haven't already — knowing your actual state of charge over a full 24-hour cycle tells you far more than any paper calculation. That said, if you're regularly getting below 30% SOC, I'd absolutely go bigger from the start rather than retrofitting later. Adding a third Fogstar Drift would be the tidiest solution given you're already familiar with that brand. Good luck with it!

@BoatMick done exactly this on the narrowboat — thought 200Ah was generous until a cloudy British weekend reminded me that "seemed plenty on paper" is just the universe's way of setting you up for a cold shower at 6am.