Powering a 3x4m garden office on solar — what size system did you end up with?

Finally pulling the trigger on a proper garden office build this spring and trying to nail down the solar setup before I commit to anything. The structure itself is sorted — 3x4m timber frame going in the corner of the garden, no mains connection (neighbour's tree makes trenching a nightmare). So it's going solar or nothing, really.

My typical loads would be a laptop, a couple of monitors, LED lighting, a small desk fan in summer, and maybe a 600W oil rad on the coldest days — though I'm a bit wary about running heating off batteries. I've been modelling it out and reckon I'm looking at somewhere around 400–600Wh of daily consumption on a normal working day, more if the rad is involved.

I'm leaning toward a 800W panel array (maybe 2x 400W on a south-facing roof), a Victron SmartSolar MPPT, and somewhere around 100–150Ah of lithium (probably a cheap Chinese LiFePO4 to keep costs down). Has anyone gone down this route with a similar-sized office and found that was enough through a UK winter, or did you end up wishing you'd gone bigger on the battery bank? Genuinely unsure whether 100Ah is optimistic for January/February.

@ForestSolar — worth being precise about what "powering" actually means here, because that phrase is doing a lot of heavy lifting in your post.

A 3×4m office running a single laptop and LED lighting is a completely different proposition to one with a monitor array, a kettle, and a fan heater in February.

My motorhome setup taught me this the hard way — I undersized badly first time round, running Fogstar 100Ah lithium paired with 200W of panels, and it simply wasn't enough once I added a second screen.

Before anyone here can meaningfully recommend a system size, you need to know:

• Your peak watt draw (list every device)
• Daily kWh consumption estimate
• Whether you need winter resilience or just April–October

A Victron SmartShunt logging your actual usage for a few weeks is worth more than any rule-of-thumb recommendation.

Good shout from @PanelLouise there. To add to that — your location in the UK matters more than people realise. A system that's perfectly adequate in Surrey will leave you short through winter in Scotland. Peak sun hours vary quite a bit across the country.

Also worth thinking about your worst-case day rather than your average. A single 400W panel and 100Ah battery might handle summer brilliantly, then completely let you down come November when you actually need the office most.

What's your intended usage pattern? Daily 9-5 kind of thing, or more occasional weekends? And are you planning any heating, because that changes the conversation entirely — resistive heating is a solar killer. Even a small oil-filled radiator will dwarf everything else in your load calculation.

Really good points already from @PanelLouise and @Chopper84. One thing I'd add that often gets overlooked at the planning stage — think carefully about your winter usage pattern, not just summer. A 3x4m office is very achievable on solar, but if you're working full days through December and January, you'll want either a decent battery bank to buffer those short gloomy days, or at least a grid-tie fallback option wired in from the start, even if you don't use it immediately. Retrofitting a consumer unit connection later is a pain. Also worth knowing your roof pitch and orientation before sizing panels — south-facing at 30-35° is ideal but not always possible on a garden structure. What direction does your building face?

Not directly related to the garden office side of things, but I've been sizing up something similar for my van build and the one thing that's tripped me up is underestimating winter output.

@Chopper84 mentions location mattering — worth actually plugging your postcode into something like PVGIS to get realistic generation figures month by month rather than relying on headline panel ratings. My Renogy panels are rated at 200W each but I'm seeing maybe 40-50W on a grey January day parked up in the Midlands.

Has anyone here gone with a hybrid inverter for a setup like this, where grid-tie might be an option later? Wondering if it's worth the extra outlay on Victron kit upfront rather than having to replace everything down the line.

Really useful thread this. One thing I haven't seen mentioned yet — think carefully about when you'll actually be using the office. If it's primarily weekday daytime working hours, your storage needs are quite different to someone doing evenings or weekends. I run a 400W panel setup with a 200Ah lithium battery in my 3x3m office and it's been fine for a laptop, monitors, and a small fan heater on the lower settings. But I'm mostly in there 9-5 Monday to Friday, so the panels are generating while I'm consuming. If you're doing more evening work you'd want considerably more battery capacity. Also worth factoring in a small electric oil radiator for winter — that'll be your biggest draw by far and might influence whether you supplement with a grid connection as backup.

Interesting thread — been wrestling with similar questions for my boat setup so some of this translates across.

One thing I'd flag specifically for a garden office: laptop and monitor loads are deceptively low but a kettle or small fan heater will absolutely murder your battery reserve. Even a 1kW heater running for a couple of hours a day in winter will dwarf everything else combined.

If you're in any doubt, get a plug-in energy monitor (around £15 from Amazon) and run it on your current home office setup for a week before you spec anything. That real-world data is worth more than any online calculator.

What battery chemistry are you leaning towards? Victron/Fogstar LiFePO4 is the obvious route but the upfront cost does change the maths on what size system actually makes sense.

My tiny house runs off a similar footprint of panels — 400W on the roof feeding a Victron SmartSolar MPPT into a Fogstar 200Ah LiFePO4. That combo handles lighting, a laptop, small screens and a decent monitor without drama.

The bit most people undersize is the battery. Panels look impressive on paper but three overcast UK days in January will drain a modest bank faster than you'd expect. I'd genuinely suggest planning for two days autonomy minimum, not one.

Also worth factoring in whether this becomes emergency backup for the main house — mine does double duty and that shaped my sizing considerably.

Really good timing on this thread @ForestSolar. One thing worth factoring in that nobody's touched on yet — shading from the office roof pitch angle itself. A 3x4m structure gives you a decent but not massive panel footprint, so losses from even partial shading hurt more than on a larger array. I'd strongly recommend string diodes or better yet individual MPPT inputs if you're fitting multiple panels. Also, don't underestimate your winter usage — a garden office in January with a kettle, monitors and a small heater chews through stored energy surprisingly fast. Size your battery capacity around your worst-case season, not your best.