Sizing a battery bank for a garden office — how much did you actually end up needing?

Finally pulling the trigger on a proper off-grid setup for my 4x3m garden office. I've been running a long extension lead from the house for two years and I'm sick of it. Planning to go with a 400W panel array (two 200W mono panels) feeding into a Victron SmartSolar 100/20 MPPT, but I'm stuck on battery sizing.

My loads are pretty modest — a laptop, a couple of USB-C monitors, a small LED lamp, and occasionally a kettle (though I know that's a nightmare for off-grid). Rough calculation puts me at around 300-400Wh per working day. I was looking at a 100Ah 12V LiFePO4, which gives me roughly 1.2kWh usable, so nearly three days of buffer — sounds fine on paper but I've never actually lived with a system like this through a UK winter.

The thing that's worrying me is November through February. Even with the MPPT doing its best, you're looking at maybe 1-2 peak sun hours on a bad week in the north of England. Has anyone found they seriously undersized their bank initially and had to upgrade? I'd rather overbuild now than be sat in the cold and dark in January with a dead bank and a deadline to hit.

@PV_Master similar situation here — garden office, fed up with the extension lead running across the lawn.

One thing I'd push back on: whatever capacity you think you need, add at least 30-40% on top. I initially thought 100Ah would cover me comfortably, but once you factor in not wanting to hammer lithium below 20% regularly, plus a couple of overcast days back-to-back, it gets tight fast.

What's your actual load looking like? Monitor, laptop, lighting, heating? That last one changes everything — a small electric heater will absolutely destroy your bank in winter.

Also worth checking whether Fogstar have any graded cells at the moment — picked up a decent 200Ah battery there significantly cheaper than retail. Paired with a Victron SmartShunt so I can actually see what's happening in real time.

What inverter/charger are you going with?

@PV_Master the EV charging angle is what caught my eye here — that changes the calculation dramatically compared to a standard office load.

I run a similar setup in Sussex and made the classic mistake of sizing for the office alone, then wondering why I'd drained my Fogstar batteries to nothing by Wednesday trying to top up my Leaf overnight.

The honest answer: double whatever you think you need, then add 20%.

My eventual setup landed on a 10kWh bank before EV charging felt genuinely comfortable rather than anxious. Victron kit throughout, which lets me monitor exactly where every amp is going — worth every penny for the peace of mind alone.

What's your typical daily mileage? That's really the starting point before you can sensibly size anything.