Sizing a battery bank for a garden office — what am I missing?

Planning a small off-grid setup for a timber garden office, roughly 12m². Not connected to the mains and the groundworks to run a cable would cost more than the solar kit, so going fully off-grid. Primarily running a laptop, a couple of monitors, LED lighting, and a small fan heater for the shoulder seasons — though I know the heater is the awkward one. Rough load estimate is around 300–400Wh/day excluding heating, which pushes toward 1–1.5kWh/day when the heater kicks in for a couple of hours.

I've been doing a fair bit of reading and the general rule of thumb seems to be 2–3 days of autonomy with 50% DoD for LiFePO4, which puts me somewhere around 6–10kWh of usable capacity depending on which estimate I use. I'm currently leaning toward a pair of Fogstar Drift 200Ah 12V batteries wired in series to give a 24V system, then stepping up to a Victron MultiPlus-II 24/3000 inverter-charger. That gives me roughly 4.8kWh usable at 80% DoD — probably undersized but it's a starting point.

Solar array is still undecided. UK irradiance being what it is, I'm thinking 1,200–1,500Wp on a south-facing roof with maybe 30° tilt, feeding into a Victron SmartSolar MPPT. The maths in winter looks painful — 1–2 peak sun hours on a bad December day doesn't go far. Has anyone actually run a garden office setup through a UK winter on solar alone, and how did you manage the heating problem specifically?

DalesSolar | Posts: 847

@RiverRunner one thing people often overlook is the difference between usable capacity and nameplate capacity. A 200Ah lithium bank genuinely gives you around 180Ah usable, but a lead-acid equivalent at the same rating? You're realistically looking at 100Ah before you start damaging the cells with deep discharge.

Also worth thinking about your worst-case winter scenario — not just daily consumption but consecutive grey days. I'd model for 3-4 days of minimal solar input in December and size your bank accordingly rather than just matching your average daily use.

What's your intended load? Laptop, lighting, and a small heater are very different propositions from each other. If you're running anything resistive for heating, that changes the calculation dramatically and you'd likely be better served by a separate solution for that.

EwanLamb | Posts: 312

@RiverRunner one thing worth adding to what @DalesSolar is getting at — don't just think about daily consumption, think about your worst case autonomy period. In the UK, you can realistically get 3-4 consecutive grey days in winter with virtually no solar gain. Your battery bank needs to carry you through that, not just overnight.

For a 12m² office I'd also factor in whether you're heating it electrically — that changes everything. A laptop and LED lighting is very manageable off-grid, but even a small fan heater will absolutely destroy your calculations.

What's your intended use — year-round or mainly spring through autumn? That makes a massive difference to how aggressively you need to size both the panels and the storage. Winter in the UK is brutal for solar yield.

DerekKnight | Posts: 1,203

Good points from @DalesSolar and @EwanLamb already. One thing I'd add that catches people out — think carefully about your worst case scenario, not your average use. January in the UK, you might get two or three decent solar days in a fortnight. I run a similar setup in the Yorkshire Dales and I sized my bank thinking "summer me" rather than "February me." Costly mistake.

Also worth considering what you're actually running in there. A monitor, laptop and LED lighting is very different from a small electric heater or a kettle. List everything with its wattage and roughly how many hours per day, then work backwards from that. What's your intended usage, @RiverRunner? That'd help folk here give you much more specific advice on panel and battery sizing.

FrankGibson | Posts: 489

Something nobody's mentioned yet — think carefully about your worst case winter week, not just daily averages. I learned this the hard way on my boat before I applied the same thinking to an EV charging setup I run off-grid.

December in the UK you might get two or three decent generation days in a fortnight. Your battery bank needs to bridge that gap, not just top up overnight.

I'd run your numbers through a tool like PVWatts or even the basic Victron MPPT calculator using your actual postcode data. When I did this properly, I realised I needed roughly 40% more battery capacity than my "average day" maths suggested.

Fogstar Drift LiFePO4 cells are worth a look for the bank itself — decent value and genuinely solid cycle ratings for a static install like yours.

AnneHenderson82 | Posts: 847

Great advice already from everyone. One thing I'd add — don't overlook your inverter's idle draw. Even when you're not actively using anything, a decent inverter sat on standby can quietly consume 10–20W continuously, which really adds up over a 16-hour winter night. Some cheaper units are worse than that.

Also worth thinking about whether everything actually needs to run through an inverter at all. Lighting, phone charging, small fans — these can often run directly from 12V or 24V DC, which is far more efficient and keeps your battery bank working harder for the things that genuinely need AC. @FrankGibson's point about worst-case winter weeks is spot on, and DC loads will help you stretch whatever capacity you've got. What inverter were you planning on using, @RiverRunner?