Powering my 12x10 garden office - is 400W of solar actually enough year-round?

Finally got my garden office build finished last month and I'm now trying to nail down the solar setup properly. The cabin runs a decent-spec laptop, a couple of monitors, LED lighting, a small fan heater for the worst of winter, and a little under-desk fridge. I've done a rough load calculation and I'm looking at somewhere around 800-900Wh per working day on average, spiking higher in winter obviously when the heater's pulling 1kW on and off.

I've got 400W of panels on the south-facing roof (two 200W monos from BougeRV), feeding into a Victron SmartSolar 100/30 MPPT, and a 200Ah 12V LiFePO4 battery from Fogstar. On paper the summer months look absolutely fine — I'm generating way more than I need and the battery barely dips. But November through February worries me. I'm in Shropshire so not the sunniest corner of England, and I'm realistically only seeing maybe 1.5-2 peak sun hours some days.

Has anyone actually run a proper working setup through a full UK winter on a similar array? I'm wondering whether I should bung another 200W panel on there now while I'm at it, or whether a small grid-tie backup — even just a basic EV charger timer into the battery — makes more sense than throwing more panels at the problem. The heater is obviously the elephant in the room and I'm half-tempted to just put it on a separate mains circuit entirely and keep the solar purely for the low-draw stuff.

MountainGeoff | 847 posts | ⭐ Trusted Member

@PanelMike congrats on the build! The honest answer is: 400W will be brilliant June through August, adequate in spring/autumn, but genuinely struggling December to February. Up here in Scotland I reckon on losing roughly 70-80% of theoretical output in midwinter compared to peak summer.

The kit you've listed sounds like maybe 150-200Wh daily consumption? That's manageable, but factor in that a grey January day might only give you 1-2 peak sun hours rather than the 4-5 you'd get in July.

My suggestion - nail down your actual battery capacity first. With decent storage (200Ah+ lithium), 400W of panels can top it up across several decent days even in winter. It's rarely about the panels alone.

What orientation is your roof facing? That makes an enormous difference to the real-world numbers.

Compo89 | 234 posts

@PanelMike worth thinking seriously about battery capacity alongside the panel wattage - people often fixate on the generation side but a decent battery bank matters just as much. In winter you might only get 2-3 usable sun hours on a good day here in the UK, so your 400W is effectively behaving like 150W for much of November through January. I'd suggest logging your actual daily consumption first using a simple energy monitor before committing to a system. If your laptop and monitors are pulling say 150-200Wh across a working day, 400W with a quality 200Ah LiFePO4 battery is probably workable spring through autumn, but you'll want a small backup like a grid hookup or generator for those grim winter weeks unless you can oversize the panels.

BreezyMechanic | 312 posts | 🔧 Regular

@PanelMike the winter months are the real test — I learned this the hard way with my shepherd's hut setup. December and January you're realistically looking at maybe 2-3 peak sun hours on a good day in the UK, so that 400W becomes roughly 800Wh on a decent day, less on overcast stretches.

What's your battery bank looking like? That's honestly where I'd focus energy (no pun intended). I run a Fogstar Drift 200Ah LiFePO4 and it's transformed how I think about cloudy weeks — storage capacity matters as much as panel wattage.

For a working office with dual monitors, I'd personally want at least 600W of panels for genuine year-round confidence, or accept you'll be topping up from the grid November through February.

Nobby43 | 156 posts

@PanelMike good shout from @BreezyMechanic about winter - that's where most people get caught out. Worth knowing that in December/January you're realistically looking at maybe 1-2 peak sun hours per day in the UK, compared to 4-5 in summer. So your 400W array effectively shrinks to about 80-100W of usable generation on a grim January day. If you're working full-time hours in there through winter, I'd seriously consider whether you can squeeze a fifth panel in. Where are you located roughly? Scotland vs south coast makes a surprisingly big difference. Also, what orientation is your roof? East-west splits can really hurt you compared to a decent south-facing pitch.