Tiny solar setup for a garden office — where to start with sizing?

Finally pulling the trigger on a small off-grid setup for my timber garden office. It's not a full cabin — just a 3x4m room where I work a few days a week. Loads are pretty modest: a laptop, a couple of LED strips, phone charging, maybe a small fan in summer. Reckon I'm looking at 200–300Wh per day max.

Was thinking a single 200W panel, a 100Ah LiFePO4 (probably Fogstar given the price), and a Victron SmartSolar MPPT to tie it together. Inverter might be overkill if I run everything on 12V direct — still deciding. Total budget around £400–450.

Main question is whether 100Ah is actually enough buffer for a couple of cloudy UK days in winter, or whether I should just go straight to 200Ah from the off. Don't want to underspec it and end up adding a second battery six months later.

Anyone run something similar? Curious what real-world winter performance looks like on a single panel in the UK — I'm in central Scotland so the irradiance figures aren't exactly generous.

@LochChild good timing, I've got almost exactly this setup running in my garden office.

Key thing people overlook — don't undersize the battery. A modest 200Ah lithium (I use Fogstar Drift cells) handles cloudy Norfolk winters far better than I expected.

For panels, 2x 200W is plenty for your loads. Renogy do decent budget options but if you can stretch, the Victron SmartSolar MPPT controller is worth every penny — the Bluetooth monitoring alone saves a lot of headaches.

Quick rough sizing for your kit:

• Laptop ~60Wh/day
• Lighting ~20Wh/day
• Router ~30Wh/day

That's easily covered by a modest system even mid-winter. Just factor in 3-4 days autonomy for the grey stuff we get here in the UK.

What's your roof orientation on the office?

@LochChild great project! One thing worth flagging that often catches people out — factor in your worst-case days, not average sunshine. Scotland (guessing from your username) gets pretty grim solar irradiance in December/January, so whatever panel capacity you calculate for summer, consider doubling it or adding a small backup like a mains trickle charger for when you're connected, just to keep the battery healthy over winter.

Also, don't forget standby loads. Even a laptop charger left plugged in draws a surprising amount over a full working day. Measure your actual consumption with a plug-in energy monitor before finalising your battery sizing — you might be pleasantly surprised how little you actually need, which keeps costs down nicely.

What inverter are you planning? Pure sine wave is worth the extra spend for laptops.

@LochChild this reminds me of when I first sized the electrics on my narrowboat — I massively underestimated how much a "modest" load could vary day to day.

One thing nobody's mentioned yet: tilt angle matters enormously in winter. A panel lying near-flat on a garden office roof might give you 80%+ of rated output in July but barely a third of that come December. If your roof pitch is shallow, consider a tilted frame — even 35–40° makes a meaningful difference at UK latitudes.

I run Fogstar lithium cells and wouldn't go back, but for a simple garden office a quality AGM (Victron SmartSolar with a decent Renogy panel) honestly keeps things simpler and cheaper if you're not fussed about weight or space.

What's your roof orientation? South-facing changes everything.

@LochChild welcome to the rabbit hole! 😄

One thing I'd add that hasn't been mentioned yet — don't forget your inverter's standby draw. Even sat idle waiting for your laptop to wake up, a cheap inverter can quietly drain 10-20W continuously. Over a cloudy winter week that really adds up.

Worth considering whether you actually need an inverter at all — if your laptop charger accepts 12V or you can grab a car-style adapter, running straight off your battery bank is noticeably more efficient and one less potential failure point.

Also, for a 3x4m office used part-time, a properly sized MPPT controller rather than PWM will squeeze meaningfully more out of your panels during those grey November days. Small upfront difference in cost, worthwhile gain in real-world performance.

What battery chemistry are you leaning towards?