Sizing a battery bank for a garden office – am I massively overthinking this?

Been planning a off-grid setup for my 12x10ft timber garden office for a few months now and I've gone down so many rabbit holes I've completely lost the plot. Currently leaning towards 2x 200Ah 12V lithium (LiFePO4) batteries wired in parallel giving me 400Ah total, paired with 2x 400W panels on a south-facing shed roof. Running a 60W laptop, LED lighting, a small 12V fan, and occasionally a monitor – so maybe 400-500Wh per day realistically.

The bit I keep going back and forth on is whether 400Ah at 12V (so roughly 4.8kWh usable at 80% DoD) is overkill for that kind of daily load, or whether I'll be grateful for it come November and December when the panels are basically ornamental. I've sized it so I could theoretically run 3-4 days without meaningful solar input, which felt sensible but now feels excessive.

I'm using a Victron SmartSolar 100/30 MPPT and a Victron MultiPlus 12/1200 inverter/charger, so the charge management side feels sorted. I did consider just doing one 200Ah battery to start and expanding later, but I've read enough threads to know that adding mismatched cells later is a headache.

Has anyone actually run a similar setup through a UK winter and kept notes on state of charge? Really curious whether people end up supplementing with a small generator or shore power hook-up those darker months, or whether decent panel sizing genuinely carries you through.

@WingAndPrayer88 snap out of the rabbit hole — done a similar build in a cabin setup and honestly 2x 200Ah 12V LiFePO4 is solid for that size.

Few things worth checking before you commit:

• What's your actual daily load? Laptop, lighting, monitor — add it up in Wh
• Solar input — south-facing roof available or are you relying on mains top-up?
• At 12V you'll want decent cable runs kept short, otherwise consider going 24V

Fogstar Drift cells are popular on here for the budget, Victron if you want bulletproof monitoring. Don't overthink the battery size — most people underestimate their charging setup and that's where it falls apart.

What inverter are you planning?

@WingAndPrayer88 honestly for a 12x10 garden office you're probably in the right ballpark with that setup. The thing that trips most people up at this stage isn't the battery sizing itself — it's not having a proper handle on their actual daily load. Before you go any further, spend 20 minutes doing a realistic load calculation. List everything you'll run, wattage, and hours per day. Kettle, monitors, laptop, lighting, phone charging — it all adds up differently depending on how you actually work. Once you've got that figure you can work backwards sensibly rather than just picking numbers that feel right. What's your typical working day looking like in there?

@WingAndPrayer88 The rabbit hole is real — I spent three months paralysed by spreadsheets before finally just building the thing for my shepherd's hut.

One thing nobody's mentioned yet: think seriously about your winter usage pattern. A 400Ah bank sounds generous until December rolls around and you're getting three hours of usable sun. I run a Victron SmartSolar with a Fogstar Drift LiFePO4 bank and the difference between a "works in August" system and a "works in January" system is often less about battery capacity and more about having decent grid-tie backup or a small genny for the dark months.

What's your planned inverter situation? That'll tell you more about whether your battery sizing is sensible than almost anything else.

@WingAndPrayer88 One thing nobody's mentioned yet — think carefully about your worst-case winter scenario rather than average usage. A grey week in January with barely any solar gain is what'll really test your system. With 400Ah LiFePO4 you've got decent headroom, but I'd strongly suggest logging your actual usage for a week or two before finalising anything. Plug in a simple energy monitor on your current setup wherever you're working now and see what you're genuinely pulling. Most people are surprised how modest a garden office actually is — kettle and monitors aside, you're probably looking at far less than your spreadsheets suggest. The anxiety of "have I got enough?" tends to dissolve pretty quickly once you see real numbers rather than worst-case theoretical ones.