Sizing a battery bank for a garden office — going round in circles, need a sanity check

Been planning a small off-grid setup for a 16x10ft shepherd's hut I'm converting into a garden office. No mains connection on that side of the plot, so fully off-grid it is. I've done a basic load calc: laptop, monitors (two 27" LG), LED lighting, a small fan heater on the coldest days, phone/tablet charging. Rough daily consumption is coming out around 1.8–2.2kWh depending on season, but winter is the scary bit with maybe 2–3 usable sun hours and the heater pulling 1kW.

Current thinking is 400W of panels (two Renogy 200W monos), a Victron SmartSolar MPPT 100/30, and either a 200Ah 12V LiFePO4 or stepping up to a 24V system with a 100Ah bank — effectively the same usable capacity but cleaner wiring at higher voltage. Fogstar Drift 24V 100Ah is on my radar. Not keen on spending Pylontech money but open to being talked into it.

The bit I keep getting stuck on is winter autonomy. If I size for 2 days of cloudy weather I'm already at ~4.4kWh usable needed, which at 24V pushes me toward 200Ah (4.8kWh). That feels right but then the recharge time after a grey week worries me. Has anyone actually run numbers on this for a UK garden office, particularly November–February? Do you just accept a small genny as backup or is there a better way to size out of that corner?

Bazza41 | 847 posts | ⚡ Solar Enthusiast

@BatteryBarry sounds like you're in the right place mate! Post your load calc and we can have a proper look. The usual culprits that catch people out with office setups are monitors (higher than people expect), and forgetting to account for laptop chargers drawing more than the laptop's rated wattage.

Also worth thinking about your worst-case winter scenario — a 16x10 shepherd's hut office in January might only see 1-2 peak sun hours on a cloudy week up north. Are you planning any backup charging, like a small generator or DC-DC from a vehicle? That can let you size the battery bank a bit more sensibly rather than going massive trying to cover every eventuality.

What panels are you thinking and whereabouts in the country are you?

GemmaFisher82 | 312 posts | ☀️ Off-Grid Convert

@BatteryBarry definitely post that full load calc when you're ready — the devil's in the detail with these things! One thing that catches people out with garden offices specifically is accounting for winter usage. If you're working in there through December/January, your solar generation drops dramatically whilst your heating load goes up. I'd suggest calculating for a worst-case winter week, not just an average. Also worth thinking about autonomy days — how many consecutive cloudy days do you need to cover without the panels doing much? Most folks aim for 2-3 days minimum. That figure has a huge impact on final battery sizing. Looking forward to seeing your numbers! 🙂

GillGibson | 1,204 posts | 🔋 Battery Nerd

@BatteryBarry one thing that trips people up at this stage — don't forget to factor in your inverter's idle draw when doing the calc. Even a quality inverter sat doing nothing can quietly chew through 20-30W continuously, which over a 10-hour day adds up to a sneaky 200-300Wh you hadn't accounted for. In a shepherd's hut setup it can genuinely skew your sizing by a meaningful margin.

Also, what's your intended usage pattern? Five days a week year-round is a very different beast to occasional weekend use, and it'll drastically affect whether you need to size for winter worst-case or can get away with something leaner. That context will help us give you proper advice once you share the full load calc.

MistyRigger | 2,156 posts | ⚡ Solar & Off-Grid Installer

@BatteryBarry welcome to the rabbit hole! One thing worth flagging before you even post the load calc — what's your winter working pattern like? A garden office setup is quite different from, say, a weekend retreat. If you're in there Monday to Friday through December and January, your solar harvest drops dramatically whilst your loads (heating, lighting, monitors running all day) stay high or increase. Plenty of folk size their battery bank around summer figures and then wonder why they're perpetually flat come November. Think about your autonomy days too — how many consecutive grey days can you tolerate before you'd want a backup option? That'll shape your bank size considerably. A small generator or even a mains hook-up as emergency backup might save you from massively over-specifying the batteries themselves.

WD40Wizard11 | 847 posts | 🔧 Hut/Boat/Office Tinkerer

@BatteryBarry running a similar setup in my shepherd's hut-turned-office — 16x12ft. Went with a 200Ah 12V LiFePO4 (Fogstar Drift) paired with a Victron SmartSolar MPPT and 400W of panels.

Honestly the thing that bit me early on was winter solar yield. Whatever your panel calc says, assume 50% of that during Dec-Jan in the UK. I spent a week in February watching the Victron Connect app with my heart in my mouth.

Also — what's your primary heating plan? That'll likely dwarf everything else in your load calc. My propane heater changed the whole equation once I factored it in properly (it runs an electric igniter and fan).

Post the full calc when ready, happy to sense-check it.