Tiny cabin build - is 400W of solar actually enough for year-round UK use?

Finally making progress on my 20m² off-grid cabin in Perthshire and I'm trying to nail down the solar setup before the roof goes on. I've been going back and forth on panel capacity and currently leaning towards 400W (two 200W panels) feeding into a 100Ah 12V LiFePO4 battery via a Victron 75/15 MPPT. Main loads are LED lighting, a 12V compressor fridge (about 45Ah/day in summer), laptop charging, and a small water pump. No inverter loads to speak of.

The honest worry is the Scottish winter. I know December and January are brutal up here — we're talking maybe 1–1.5 peak sun hours on a good day, and plenty of weeks where it barely gets above the horizon. Even with the panels tilted steeper for the low sun angle I'm struggling to see how 400W pulls enough in to keep the battery topped up without a backup source. Has anyone run similar numbers through PVGis for Scotland and found it genuinely viable, or is it basically guaranteed you'll need a generator or wind turbine top-up?

I do have a small Jackery 500 kicking about that I could use as a bridge, and there's a decent burn nearby so micro-hydro is theoretically on the table — though that feels like a whole other project. Wondering if the simpler fix is just bumping to 600W of panels and a bigger battery, maybe 200Ah, and accepting some generator use in the worst weeks rather than overcomplicating things.

Has anyone here actually lived in or used a cabin at this latitude through winter on solar alone, or is some form of backup essentially non-negotiable above the central belt?

Chopper62 | 847 posts | ⚡ Solar Enthusiast

@HamishLee Perthshire in winter is the real challenge here - you're looking at potentially only 1-2 peak sun hours daily from November through January. I'd honestly say 400W is workable in summer but you'll struggle badly in the darker months without a solid backup strategy.

The bigger question nobody's asked yet - what's your battery bank size? 400W of panels feeding a modest battery is far more limiting than the panel wattage itself suggests. I run 600W up near Stirling and still fire up my generator occasionally in December.

What's your expected daily consumption? That number will tell you more than anything else whether 400W cuts it. Heat, lighting, and devices all add up fast in a small cabin during winter.

CamperAndrea | 312 posts | 🚐 Van & Cabin Builds

@HamishLee @Chopper62 makes a fair point about winter, but I'd add that it's not just about total wattage — panel orientation matters enormously in Scotland. A lot of folk stick panels flat on a shallow pitched roof and lose a huge chunk of winter output when the sun barely gets above 15° elevation.

If your roof allows it, consider a steeper tilt or even a small ground-mounted array you can adjust seasonally. I went through two winters underestimating this before I sorted mine properly.

Also, what's your battery capacity looking like? 400W feeding into a modest bank in November is a very different conversation to the same panels with proper storage behind them. Perthshire gets some brutal overcast spells — you really want enough capacity to bridge 4-5 grim days comfortably.

MandyRoss | 1,204 posts | 🏕️ Static Caravan & Off-Grid Systems

Running a static caravan in Scotland year-round, so directly relevant experience here. The number people miss is peak sun hours — Perthshire in December averages roughly 0.8–1.2 PSH daily. With 400W nominal, you're realistically generating 320–480Wh on a decent winter day, probably less with any shading or panel soiling.

My practical recommendation: size your battery bank to cover 3–4 days autonomy, not just overnight. I run a Victron SmartSolar MPPT with a 200Ah Fogstar Drift LiFePO4 bank, and even that feels tight November through January.

Also worth considering panel orientation — a steeper tilt angle (55–60°) suits Scottish winter sun elevation far better than the typical 35° you'd use down south. Squeezes meaningfully more generation out of low-angle winter light.

400W can work, but only with disciplined

Chunk | 2,341 posts | 🔋 Off-Grid Obsessive

@HamishLee 400W is honestly fine as a starting point for summer, but Perthshire will properly humble you come November. What nobody's mentioned yet is roof orientation and pitch - a steeper tilt (50-60°) actually helps you squeeze more out of weak winter sun and sheds snow better too. Also worth factoring in your shading situation; even partial shadow from nearby trees kills output dramatically.

My honest suggestion: design your battery bank for 3-4 days autonomy rather than trying to solve winter purely with extra panels. A small backup like a wind turbine or even a petrol genny for occasional winter top-ups is far more cost-effective than doubling your panel count for those dark December weeks.

What's your planned battery capacity so far?

RoundTuit | 847 posts | 🏡 Garden Office & EV Charging

@HamishLee The Perthshire location is doing a lot of work here. pvgis.ec.europa.eu is your friend — plug in your exact coordinates and you'll get monthly irradiance figures. December/January up there you're looking at perhaps 0.5–0.8 peak sun hours on bad stretches.

400W panels producing ~1.5kWh/day in winter versus potentially 8–10kWh in summer is a brutal ratio. My garden office setup taught me that battery capacity matters far more than panel wattage once you hit November.

Practically speaking: what's your load profile? A cabin doing LED lighting, phone charging, and a laptop is a completely different beast to one with an inverter running a kettle. Size your battery bank around 3–4 days autonomy for winter, then back-calculate what panels can realistically recharge it in December.

IanPearce56 | 312 posts | 🏕️ Static Caravan

Got a static caravan setup myself and the December/January figure is what catches people out every time. You're not just looking at panel wattage — it's about days of autonomy in your battery bank when you get a solid week of overcast.

Perthshire in January you might see 0.5-1 peak sun hours. Your 400W becomes 200-400Wh on a good day. If you're running even modest loads that's gone by evening.

My suggestion: design the roof for 600-800W capacity now even if you only fit 400W initially. Panels are cheap, structural changes aren't. And pair it with a decent MPPT — Victron SmartSolar handles the low-light conditions noticeably better than the budget alternatives I've tried.

What's your battery capacity looking like?

LesPhillips | 156 posts | 🏡 Garden Office & Shepherd's Hut

Running two separate setups here — garden office and a shepherd's hut — so I've got a decent comparison. The hut has 400W and genuinely struggles November through February even with minimal loads. The office has 600W and a bigger battery bank (Fogstar 200Ah) and it's a different world.

@IanPearce56 is right to flag winter — that's where Perthshire will really bite you. I'd seriously consider whether 400W is your starting capacity and leave roof space for expansion. Victron MPPT gives you good visibility of what you're actually harvesting, which quickly tells you whether you need more panels.