Powering my garden office off-grid — is 400W of solar + 100Ah lithium enough year-round?

Finally got the go-ahead from the missus to build a proper garden office at the bottom of our plot in Shropshire. Rather than dig a 30-metre trench back to the house, I'm seriously considering going fully off-grid for it. Feels like the right call given how much this community has taught me about solar over the years.

My rough plan is two 200W panels on the south-facing roof (it's a 4x3m cabin, so space is limited), a 100Ah lithium LiFePO4 battery, and a Victron MPPT 75/15 controller. For loads I'm looking at a laptop, a couple of monitors, LED lighting, a small fan in summer, and maybe a 300W oil-filled radiator on a thermostat for the coldest months. Daily consumption I've estimated at roughly 400–500Wh on a typical working day.

The winter months are what worry me. I'm at roughly 52°N and I know December and January can be brutal for generation — we're talking maybe 1–2 peak sun hours on a decent day, worse on overcast weeks. A 100Ah (128Wh usable at 80% DoD on 12V... wait, that's 960Wh, sorry) battery gives me a reasonable buffer, but I'm wondering whether I should upsize to 200Ah now rather than regret it later.

Has anyone run a similar setup through a full UK winter, particularly in the Midlands or further north? Did you find a small generator useful as a backup, or did you just accept the odd dead day and work from the house? Really keen to hear real-world experience rather than theoretical calcs.

GlenKelly | 847 posts

@JoeFox55 Shropshire's a tricky one — you're not exactly Cornwall for winter sun! Honest answer: 400W and 100Ah might scrape through summer, but December/January will likely leave you frustrated. Those months you're realistically looking at 1-2 peak sun hours on a good day.

I'd strongly suggest bumping to at least 600W panels if roof space allows, and honestly 200Ah of usable lithium gives you proper buffer for those grey weeks we get. Also worth thinking about what you're actually running — laptop and LED lighting is very different from a monitor setup, space heater, or kettle.

What's your heating plan? That's usually the killer for small systems. An oil-filled radiator will absolutely flatten a 100Ah battery overnight. A small infrared panel on a thermostat is far kinder to your setup.

CopperWarden | 1,243 posts

@JoeFox55 @GlenKelly makes a fair point about winter, but I'd frame it differently — it's not that your system won't work, it's about managing expectations for those grim November-February stretches.

The real question is what you're actually running. A laptop, monitor, LED lighting and phone charging? You'll likely be fine most of the year. Add a kettle, electric heater or anything with a heating element and that 100Ah will vanish sharpish.

My honest suggestion: budget for a small backup charger from the mains now, even if you don't use it often. A 20-30m cable run occasionally beats scrambling when you've got a client deadline and three days of solid cloud cover.

What's your typical daily watt-hour estimate? That's really where this conversation needs to start before anyone can give you a straight answer.