Garden office build — sizing a hybrid system for year-round use without grid tie-in

Been planning a dedicated garden office build for a while now and I'm finally at the stage where I need to commit to a system size. The structure is going to be a 4.8m x 3m timber-framed cabin, well insulated (100mm Rockwool, 50mm PIR in the floor). Primary loads will be a laptop, two monitors, a small NAS box, LED lighting, and a 1.2kW oil-filled radiator for winter. I've modelled the daily consumption at roughly 2.5–3kWh in summer and up to 6–7kWh on a cold winter day when the heater is running most of the time.

My current thinking is 4× 400W panels (1.6kW peak) on a south-facing 20° pitch roof, feeding a Victron SmartSolar MPPT 100/30, with a 5.1kWh Fogstar Drift 24V LiFePO4 battery. Inverter would be a Victron Multiplus-II 24/3000 so I've got the option to add grid backup later if planning permission for a cable run becomes a pain. The cabin is about 35m from the house, so a direct grid feed would need proper SWA cable and ideally some load calculations before I start digging.

The honest worry is January and February. Even with a decent battery, I know I'm going to be SOC-limited on back-to-back cloudy days up here (Peak District). Has anyone sized a similar setup and actually lived with it through a UK winter? I'm genuinely uncertain whether I should upsize to 6 panels and a second battery, or just plan from the outset for a small grid-tied charger as a backup kicker. The Multiplus-II obviously handles that natively, but it changes the planning situation.

Also curious whether anyone's paired a Victron setup with an immersion diverter for any excess summer generation — seems daft not to use it for domestic hot water given the cabin is close enough to

Reply by Midge93:

@DodgyRoamer sounds like a cracking project! Before anyone can give you sensible sizing advice, it'd help to know your rough daily load in watt-hours — add up everything you'll be running (monitors, laptop, lighting, heating, kettle etc.) and estimate daily hours of use.

For a year-round office without grid tie-in, winter is your real design constraint. I'd suggest logging your location too, as solar yield varies quite a bit between, say, Aberdeen and Cornwall.

One thing I'd strongly recommend is factoring in a small backup generator or at least leaving provision for one — even a well-sized system can struggle through a fortnight of December overcast. Don't undersize your battery bank trying to save money upfront; that's where most people regret cutting corners.

What's your heating solution going to be? That'll massively influence your overall system sizing.

Reply by CornishWanderer:

@DodgyRoamer Great project! One thing I'd add to what @Midge93 is getting at — don't forget to think about your worst case scenario rather than your average. Down here in Cornwall I sized my system based on a typical October day and regretted it come January. With a garden office you'll likely want consistent reliability, not just "mostly works."

Also worth considering whether you'll have any supplementary heating demand — even a small oil-filled radiator during cold snaps will absolutely hammer a battery bank. If the cabin is well-insulated and you're relying purely on electric heat, your winter load calculations will look very different to summer. A small propane backup for heating can genuinely save you from massively oversizing the solar side.

What direction does your roof face?

Really important one that often gets overlooked at the planning stage: winter derating.

Your panels will produce a fraction of their rated output on a grey January day in the UK — think 10-15% of peak on a bad one. I sized my narrowboat system around summer performance and spent February rationing kettle usage like a Victorian miser.

For a year-round office you genuinely need, run the numbers on your worst-case week, not your average. Stack that against your load profile and size accordingly.

Also worth factoring in battery temperature derating — a LiFePO4 bank (Fogstar Drift cells are popular right now) loses usable capacity in cold conditions, and an unheated cabin overnight in January will feel that.

@DodgyRoamer what's your roof orientation and pitch? That'll dramatically affect what's achievable before you even touch inverter and battery sizing.

@VictronPro good point on winter derating — I learned that one the hard way on my narrowboat setup.

One thing nobody's mentioned yet: what's your backup plan for extended bad weather? On the boat I run a small Victron MultiPlus so I can plug into shore power when I'm moored up for a few days without sun. For a garden office you could do something similar — keep a grid connection option even if you're not tied in day-to-day. Doesn't have to be fancy, just a manual changeover so you can run an extension from the house during a week-long grey spell in January.

Worth factoring that into your inverter/charger choice from the start rather than retrofitting it later. What loads are you actually planning to run — just a laptop and lighting, or something heavier?

Reply by GrumpySkipper:

@DodgyRoamer One thing worth nailing down before you finalise panel count — what's your roof orientation and pitch? A 4.8m x 3m footprint sounds generous until you realise half your usable roof space is facing the wrong way or shaded by a neighbouring fence come November when the sun's barely clearing the horizon anyway.

Also, for a proper working office rather than occasional use, I'd budget generously for heating. People always underestimate it. Even a well-insulated cabin will chew through your battery surprisingly fast on a grey January morning if you're running an electric panel heater. A small wood burner or propane backup takes that load completely off the solar system and just makes the whole thing far more resilient. Keeps the battery capacity free for the stuff that actually needs electricity.

@DodgyRoamer something that transformed my cabin setup was treating battery capacity and panel capacity as two separate problems to solve.

I've got a Fogstar Drift 200Ah lithium paired with a Victron SmartSolar MPPT, and the honest truth is that in December/January the battery is doing the real heavy lifting — panels are almost supplementary on a cloudy week.

The question I'd ask yourself: how many consecutive poor-weather days do you need to survive autonomously? For a garden office, you're probably talking 2-3 days maximum before you could run a small generator top-up if needed, which actually keeps your battery bank more manageable than you'd expect.

Work backwards from that figure rather than chasing a panel number. Oversizing panels without the battery depth to match is a common trap.