Sizing a battery bank for a garden office — 3kWh enough or kidding myself?

Finally pulling the trigger on a proper off-grid setup for my 6x4m garden office. Currently running an extension lead from the house which is embarrassing and I want rid of it. Typical daily load is around 1.2–1.5kWh — laptop, monitors, a small fan heater on low for an hour or so in winter mornings, and a mini fridge running constantly. No EV charging here, that's a separate beast.

I'm looking at a 3kWh lithium bank (probably Fogstar Drift 200Ah 12V or step up to a 24V system with two in series) paired with around 600W of panels. Victron MPPT and a Multiplus 12/3000 or 24/3000 — haven't committed to voltage yet. My concern is winter: three or four days of grim UK weather, loads climbing a bit with the heater, and I'm suddenly staring at a flat battery and a very cold office.

Has anyone actually run a garden office through a UK winter fully off-grid, no grid tie? Wondering if 3kWh is just optimistic and I should be looking at 5–6kWh from the start. Retrofitting extra batteries later is always more painful than doing it right first time — learned that lesson on my motorhome build.

Also curious whether people are going 12V or 24V at this sort of scale. The cable runs aren't huge but I'd rather not sweat the wire sizing if 24V makes life easier.

Hey @WhatsAFuse65, looks like your post got cut off before you listed your loads — worth completing that as it'll massively affect the advice you get!

That said, 3kWh is a reasonable starting point for a typical garden office setup, but honestly it depends heavily on whether you're running anything with a heating element. A kettle or small fan heater will chew through capacity shockingly fast compared to a laptop and some LED lighting.

One thing people often overlook is the usable capacity — a 3kWh lithium bank gives you roughly 2.8kWh usable, whereas lead-acid you're looking at maybe 1.5kWh before you're doing damage. Makes a big difference to your real-world figures.

Finish listing your loads and I can give you a much more useful steer! ⚡

VanBarry | 847 posts

@WhatsAFuse65 you've cut off your typical daily load there, mate! Finish that sentence and you'll get much more useful answers from folk here.

That said, 3kWh can be perfectly adequate for a garden office, but the devil's in the detail. A laptop and monitors all day is very different from running a fan heater or small air-con unit, which will absolutely chew through 3kWh before lunchtime.

Worth also considering your solar input — a battery number in isolation doesn't mean much without knowing how it's being topped up. What's your roof/panel situation looking like?

Also, what inverter size are you planning? That'll partly dictate what loads are even feasible.

@WhatsAFuse65 3kWh could be fine or completely insufficient — impossible to say without knowing your loads, as others have noted.

What I'd add from running a shepherd's hut off-grid: people consistently underestimate two things —

1. Heating/cooling — a small fan heater or portable AC will absolutely batter a 3kWh bank in no time
2. Winter solar yield — you might size fine for July and find yourself running on fumes by November

My narrowboat taught me to always overspec the battery and underspec your expectations of the panels. If you're genuinely considering 3kWh, I'd be looking at Fogstar Drift LiFePO4 cells minimum — at least you're not losing 20% to Peukert losses like lead acid.

Post your load list and whether you're heating the space. That's the critical question nobody's answered yet.

EwanLamb | 234 posts

@WhatsAFuse65 Looks like everyone's spotted the cliff-hanger! While you're editing that post, also worth mentioning whether you're planning any heating in there — that's often the killer for garden office battery sizing. A small oil-filled radiator or fan heater will absolutely obliterate a 3kWh bank on a grey winter's day, whereas if you're just running a laptop, monitor, and some LED lighting, 3kWh is genuinely comfortable with headroom to spare. Also worth thinking about your worst-case scenario: consecutive overcast days in November means your solar isn't topping things back up reliably. What panels are you pairing with the battery?

SunnyFisher | 312 posts

@WhatsAFuse65 Not to pile on, but that cut-off is doing my head in — it's like a cliffhanger on a soap opera except instead of "who shot JR" it's "what's his kettle wattage."

That said, from my narrowboat experience: 3kWh sounds reasonable until winter rolls in and suddenly you've got two cloudy weeks, the heating's running overtime, and you're back to running an extension lead with extra shame attached.

What inverter are you planning? And are you factoring in usable capacity — because with LiFePO4 you're generally fine at 80-90%, but if someone's talked you into AGM you'll want to halve that figure pretty sharpish. Fogstar or a Victron SmartShunt would at least tell you exactly how badly you've undersized it. 😄