How much battery capacity do I actually need for a garden office?

Been going back and forth on this for weeks and I'm completely lost. The office is a 20m² insulated log cabin — planning to run a laptop, two monitors, a small desk lamp, Wi-Fi router, and a mini fridge. Rough estimate puts me around 400-500Wh per day in summer, probably more like 600-700Wh in winter with a small heater thrown in occasionally.

I keep seeing the "take your daily usage and multiply by 2-3 for days of autonomy" rule, which would put me at a 1.5-2kWh usable capacity minimum. Looking at Fogstar Drift lithium batteries — the 100Ah 12V gets me 1.2kWh usable, so I'm wondering whether to go 12V/200Ah or jump straight to a 24V system for a garden office that isn't exactly a mobile install.

Does anyone actually size for worst-case winter days, or do you just accept you'll supplement with a hook-up or small genny during a bad week in January? Curious what people with similar permanent garden office setups actually landed on rather than the theoretical numbers.

@Bomber nothing like spending weeks agonising over battery size only to end up buying twice what you calculated anyway — ask me how I know 🚤

Rough rule of thumb: add up your wattage, multiply by daily hours, double it for depth-of-discharge headroom, then double it again because you always add "just one more thing."

For your load list you're probably looking at 400–600Wh actual daily draw — so realistically a 100Ah 12V LiFePO4 (Fogstar do decent value ones) gets you through a cloudy day, 200Ah gives you proper peace of mind.

Pair it with a decent MPPT — Victron SmartSolar is worth every penny — and a reasonably sized panel array and you'll be sorted.

Don't forget the mini fridge is the sneaky power vampire in that list.

@SussexBoater speaks from painful experience I'm sure — on the narrowboat I did exactly the same calculation twice and still ended up panic-buying a second Fogstar 100Ah at 11pm on a Tuesday.

Bomber, rough rule of thumb: add up your watt-hours per day, double it for a 50% depth-of-discharge buffer, then add another 20% because you will find something else to plug in by month two. For your load list you're probably looking at 200-300Ah at 12V as a sensible starting point, but honestly a Victron SmartShunt will tell you more about your actual usage in a fortnight than any spreadsheet ever will.

@Bomber Happy to help you work through this properly. First step is to tot up your actual watt-hours — roughly speaking a laptop draws 45-65W, each monitor around 20-30W, router 10-15W, lamp maybe 10W, and a decent mini fridge 30-50W averaged out over the day. If you're working an 8-hour day that's probably 600-900Wh of actual consumption.

Now here's the critical bit people miss — you want usable capacity, not nameplate capacity. With LiFePO4 you can use around 80-90% of stated capacity, but with lead-acid it's more like 50%. Factor in a couple of cloudy days of autonomy and you're looking at somewhere between 2-3kWh as a sensible starting point. What battery chemistry are you leaning towards? That'll change the recommendation considerably.