Powering my 6x4m garden office - is 400W of solar overkill or just right?

Finally getting round to properly sorting the electrics in my garden office and trying to nail down the right size system before I spend any money. The office is about 6x4m, timber frame, and I'm in it most weekdays for work - laptop, a couple of monitors, a small desk lamp, and a mini fridge. Occasional power tool charging too. I've done a rough load calc and reckon I'm pulling maybe 300-400Wh on a typical day, though obviously winter is a different story.

I've been looking at two 200W panels (so 400W total) feeding into a 40A MPPT controller and a 200Ah lithium battery (one of the Fogstar Drift 12V units). That comes out at 2.4kWh usable which feels like a reasonable buffer, but I'm second-guessing myself on whether 400W of generation is enough to keep that topped up through November and December when I'm up in the Midlands and we get maybe 1-2 peak sun hours on a good day.

Has anyone actually run something similar through a full UK winter without just giving up and running an extension lead from the house? Wondering whether I should be looking at a small wind turbine as a supplement or whether that's more hassle than it's worth. Roof orientation on the office is roughly south-southwest which should be decent at least.

LauraCole | 847 posts

@VivaroSolar 400W is a reasonable starting point but it really depends on what you're running in there. The big unknown for a garden office is heating — if you're relying on an electric panel heater or similar during winter, that'll absolutely dwarf everything else combined and no realistic solar setup will cover it on its own.

What's your battery capacity looking like? Solar panel wattage is almost meaningless without knowing storage, especially in a UK winter where you might get genuinely miserable generation for days at a stretch.

Also worth considering: are you working full-time in there or just occasional use? That changes the calculus quite a bit. Give us a rough list of your actual loads and we can work backwards to something sensible rather than guessing! 🙂

MistyFisher | 1,203 posts

@VivaroSolar Worth thinking about your winter usage too - that's where most people come unstuck. Up here in the UK, a 400W array in December/January might only give you 1-2 peak sun hours on a good day, so realistically you're looking at 400-800Wh on a decent winter day, possibly much less. If you're heating with an electric radiator or running a decent monitor setup through the shorter days, that can disappear quickly. What's your battery storage looking like alongside the panels? The panels are only half the equation really. Also, are you planning to be in there full-time or just occasional use? That changes everything in terms of whether 400W is adequate or whether you'd be better off going to 600W from the outset while you're doing the install anyway - the marginal cost difference isn't huge.

MV_Marine | 312 posts

@VivaroSolar What appliances are you actually planning to run though? That's the bit that's missing here. In my tiny house setup I thought 400W would be plenty until I added a small fan heater on a thermostat and suddenly my numbers looked very different.

Also worth checking your panel orientation and any shading — I dropped from 4 hours peak sun to barely 2 once I factored in a nearby fence line in winter.

Have you looked at a Victron SmartShunt to monitor actual consumption before committing? Even running it on mains temporarily to log real usage for a week would tell you more than any calculator. What battery capacity are you planning alongside the panels?