Anyone else gone overboard on battery capacity "just in case"?

Started with a 100Ah Fogstar Drift LiFePO4 in the shepherd's hut last spring. Told myself it was plenty. Six months later I've got 300Ah sitting under the bench and I'm still eyeing up another 100Ah because "winter's coming."

Running a 12V system with a Victron SmartSolar 100/30 and about 400W of panels. Realistically the hut only pulls maybe 20-30Ah a day — kettle, a few lights, phone charging, occasionally a small fan heater for an hour. Even in December I've never actually bottomed out below 60% SOC.

So at what point does it become diminishing returns? Is there a sensible rule of thumb — like X days of autonomy is enough and anything beyond that's just comfort buying? Curious whether anyone's done the maths properly rather than just panic-buying cells like me.

PeteDixon89 | 847 posts | Yorkshire

@BirchLover Ha, the classic "just in case" spiral! I did exactly the same with my narrowboat setup. Started sensible, ended up with 400Ah and honestly I sleep better knowing I've got headroom on cloudy November weeks.

The thing nobody tells you is that running LiFePO4 batteries at 20-80% rather than hammering them to the limits genuinely extends their lifespan massively, so having "too much" capacity actually means you're cycling them more gently anyway. You can almost justify the extra bank as a longevity investment rather than paranoia!

That said, my wife's started calling the battery compartment "Pete's emotional support cupboard" so maybe I need to have a word with myself. 😅

What's your solar input looking like? Sometimes that's where the real bottleneck is rather than storage.

SamWhite | 234 posts | Shropshire

@BirchLover Guilty as charged! Went from 200Ah to 560Ah over about eight months in my cabin build. The thing is, there's always a rational justification isn't there - bad weather forecast, a mate coming to stay, wanting to run the kettle without checking the state of charge first...

Honestly though, I've stopped apologising for it. Batteries don't wear out from sitting full, and that extra headroom means I'm genuinely not stressing through January when we get four hours of grey nothing.

The real question is whether your solar input can actually keep up with 400Ah. I found expanding panels was the natural next step after the battery creep, and then suddenly you need a bigger controller...

It's a wonderfully expensive hobby we've got ourselves into. 😄

HalfAJob53 | 412 posts | Derbyshire

Ha, you lot are amateurs 😄 I told my wife the 200Ah setup was "definitely final" approximately four times. Currently sitting at 720Ah and I've just been browsing the Fogstar site again "just to look."

The thing nobody warns you about is that more capacity doesn't cure the anxiety - it just shifts the threshold. Used to panic when I hit 50% SoC, now I panic at 30%. The goalposts move every single time.

@BirchLover that extra 100Ah will absolutely happen, best just accept it now and save yourself the internal debate. At least with LiFePO4 the usable capacity is genuine, unlike the lead acid days when you were kidding yourself half the time anyway.

My genuine advice - work out your actual worst-case daily usage first, then buy. I did it backwards obviously.

SolarGaz | 1,203 posts | Array

My garden office Victron BMV-712 has a "days since last reached 100% SOC" counter — currently reads zero because I panicked and bolted on another 100Ah Fogstar last Tuesday "just in case it gets cloudy."

It's been sunny all week. 😐