Went solar on my garden office last spring — here's what actually worked (and what didn't)

After months of lurking and reading threads on here, I finally pulled the trigger on a proper off-grid setup for my 4x3m garden office in Derbyshire. Ended up going with a single 400W panel on the roof (Risen RSM40-8-400M), a Victron SmartSolar 100/30 MPPT, and a 200Ah LiFePO4 battery from Fogstar. Total spend was just under £900 including cable, fusing, and a Victron BMV-712 to keep an eye on the state of charge.

Day-to-day it handles my laptop, a couple of LED strips, phone charging, and a small USB-C monitor without any bother. I was pulling maybe 150–200Wh on a typical working day. Where it fell over was November and December — three or four gloomy days in a row and I was down to 20% SoC faster than I'd expected. I'd underestimated just how dire Derbyshire winters are for solar yield. Added a 30m armoured cable from the house as a backup feed into a Victron Orion-Tr Smart DC-DC, which sorted it.

The one thing nobody really warned me about was cable voltage drop over longer runs. I used 6mm² for the panel-to-controller run (about 8 metres on the roof) and I'm glad I didn't go smaller — even so I can see a few percent drop on very bright days when the panel is pushing close to its rated output.

Has anyone else found a smarter way to handle the winter deficit without running a backup mains feed? Curious whether a second panel would actually make a meaningful difference in December or whether you're just doubling down on a losing battle when the sun barely clears the rooftops.

@SilverRanger nice write-up, curious what you ended up with for storage? Running a similar setup at my cabin and the battery choice was honestly the biggest headache. Went with Fogstar Drift LiFePO4 in the end and it's been solid — way better than the AGM I had before.

Also, Derbyshire winters are no joke for solar yield. Do you have any tilt adjustment on that panel or is it fixed? I found even bumping mine up to a steeper angle from October onwards made a noticeable difference in the shorter days.

@SilverRanger Derbyshire's a tough testing ground — those grey November weeks will really expose any undersizing decisions you made in spring when everything looked rosy.

Been through exactly this cycle on the narrowboat. First winter I was convinced my storage was adequate, then December arrived and I realised I'd basically designed a summer toy.

If you haven't already, I'd strongly recommend oversizing your battery bank beyond what feels logical. Running a Fogstar Drift 100Ah LiFePO4 here — genuinely solid bit of kit for the money — but I wish I'd gone 200Ah from the start.

The panel-to-storage ratio matters more than most people admit. On cloudy days in the Peak District you might get 40-60W effective output at best. Your battery needs to carry you through two or three of those back-to-back without drama.

What inverter did you end up with?