Tiny cabin build - how much solar is actually enough for year-round UK use?

Finally pulling the trigger on a small off-grid cabin in the Scottish Borders. Footprint is about 20m², single room, no mains connection whatsoever. Planning to use it as a proper weekend retreat plus occasional longer stays through winter.

Current thinking is 2x 200W panels on a south-facing lean-to roof, feeding into a Victron SmartSolar MPPT 75/15, with a pair of Fogstar Drift 100Ah 12V LiFePO4 batteries in parallel. Loads are modest — a few LED lights, a 12V compressor fridge, phone/laptop charging, and a small inverter for the odd tool. No immersion, no electric heating.

The bit I'm unsure about is whether 400W of panels is genuinely enough from November through February up here. I've done the PVGiz estimates and they're... not exactly confidence-inspiring for mid-winter Scotland. Wondering if I should go to 600W now rather than retrofitting later, even if the charge controller would need upgrading.

Has anyone actually run a setup like this through a Scottish or northern English winter? What did your real-world generation look like in December/January, and did you end up with a generator as backup regardless?

@TIW_Power Scottish Borders is going to punish you November through February — I'm in a static caravan setup and even with decent south-facing panels, December irradiance is brutal.

For year-round UK use in that location I'd be thinking minimum 800W-1kW of panels and honestly don't undersize the battery. I run a Victron SmartSolar MPPT and the difference proper charge management makes in low-light conditions is significant.

A few things worth considering:

• Panel tilt angle matters more in Scotland — steeper pitch (50-60°) extracts more from low winter sun
• Budget for a small generator backup, not as primary but for the inevitable week of solid overcast
• Fogstar Drift LiFePO4 cells offer decent value if you're building your own battery bank

What loads are you actually planning to run? That changes everything.

@TIW_Power Scottish winters are genuinely brutal for solar — I've got a shepherd's hut setup and November/December irradiance is maybe 15-20% of peak summer output.

For a 20m² cabin used year-round, I'd size for worst-case December, not average annual. Rough rule of thumb for Scotland: assume 1-1.5 peak sun hours per day in winter.

My practical suggestion:

• 600-800W of panels minimum (Renogy or similar)
• 200Ah+ LiFePO4 (Fogstar Drift is solid value)
• Victron MPPT — the SmartSolar range with load output monitoring is worth every penny for understanding your actual consumption patterns

Don't skimp on battery capacity thinking panels will compensate. In December they simply won't.

Also consider panel tilt angle — steeper is better for low winter sun, roughly 55-60° for your latitude rather than the standard 30°.