Sizing a battery bank for a garden office — did I massively overkill it?

So I've just finished wiring up my 10m² garden office and I'm second-guessing the battery side of things. I went with 2x 200Ah 12V lithium (LiFePO4) cells wired in series for a 24V/200Ah bank, fed by 3x 200W panels on a 40A MPPT controller. On paper that's about 4.8kWh usable, which felt sensible when I was planning it out.

Day-to-day I'm running a laptop, a couple of monitors, some LED lighting, and a small 400W ceramic heater for the worst of the cold mornings. Rough daily load is probably 600-800Wh in summer, maybe nudging 1.2-1.5kWh on a grey January day when the heater's working hard. Even being pessimistic about that, the bank feels like it could cover three or four days without any meaningful solar input.

I was originally worried about back-to-back cloudy days in winter, which is why I sized it the way I did. But now I'm wondering if 200Ah at 24V is genuinely justified, or if 100Ah would have been perfectly fine and I've just spent an extra £400 for very little real-world benefit. Has anyone else gone through this and landed somewhere sensible?

@24V_Geek what's your daily load like in kWh? That's really what determines whether 200Ah at 24V (so ~5kWh usable) is overkill or not.

For context — I've been trying to work out sizing for EV charging from an off-grid setup and the numbers get wild very quickly. Even a modest 3.6kW charge session would flatten a bank like yours in under two hours.

For a garden office though, are you running anything heavy? Heat pump, monitors, kettle? Or mostly just laptop/lighting/router type stuff? Because for the latter, 200Ah at 24V sounds pretty reasonable to me — maybe even on the smaller side if you want a couple of cloudy days of autonomy.

What solar input are you running with it? That gap in your post got cut off before you finished the spec.

@24V_Geek what panels are you running with it? That'll change the answer quite a bit.

For context, my shepherd's hut setup is similar — 24V/200Ah Fogstar Drift bank — and for basic office use (laptop, monitor, a few lights, phone charging) I rarely dip below 80% SOC even in January. So if that's your use case, yes, probably overkill.

Where it does make sense to have the extra capacity:

• Running through cloudy UK winters without a generator backup
• If you're ever tempted to add a kettle or small heater
• Longevity — LiFePO4 lasts far longer if you're not hammering the depth of discharge

Honestly "overkill" in battery sizing is rarely something you regret long-term. Undersizing is the expensive lesson.

Really depends on your usage patterns @24V_Geek. For a garden office, 5kWh usable is actually pretty reasonable if you're running a laptop, monitors, lighting, and maybe a small heater through winter. The "overkill" question really hinges on how many cloudy days you're planning to ride out without grid backup. Two days of autonomy in December in the UK is no joke — I've seen people with seemingly massive banks genuinely struggle through a grey January week. What's your solar input situation? If you're undersized on panels, all that lovely battery capacity won't replenish fast enough anyway. Also worth checking your BMS can handle your peak draw comfortably — sometimes people spec the cells well but the BMS becomes the weak link. Wouldn't call it overkill personally, more like sensibly cautious for a working office environment.

@24V_Geek honestly 5kWh usable for a garden office is fine, probably not overkill at all. The question nobody's asked yet — what's your winter plan? Short days, panels barely producing, you might be pulling that bank down pretty hard across a grey January week.

That's where people get caught out. Summer it'll be grand, winter is the real test.

Also — are those 200Ah cells genuine capacity or did you go budget brand? Because there's a big difference between Fogstar/Eve cells and some random AliExpress thing that's "200Ah" on the label and 160Ah in real life.

What BMS are you running with it? Series 24V builds can be fussy if the BMS isn't matched properly. Had a similar setup on my narrowboat and the BMS was the weak link, not the cells.