Anyone else running a Victron MPPT in a garden office? Sizing questions inside

Finally got round to wiring up my garden office properly after two years of running an extension lead from the house (embarrassing, I know). Currently got a single 200W panel on the roof feeding into a Victron SmartSolar 75/15 with a 100Ah Fogstar Drift LiFePO4. It's handling my monitor, laptop, and a small desk lamp no bother through summer.

Problem is winter's coming and I'm already seeing the MPPT throttle back massively on overcast days — getting maybe 15–20W on a grey afternoon when I need closer to 80W sustained. I'm thinking about adding a second 200W panel, but my roof space is awkward and they'd likely end up on slightly different orientations (one south-facing, one more south-west).

Has anyone run mismatched orientations into the same MPPT? I know in theory you lose efficiency when one panel shades or underperforms the other, but I'm wondering if it's actually noticeable in practice or if it's one of those things that sounds worse than it is. Would a second controller make more sense — maybe a cheap Renogy one just for the SW panel?

Also curious whether 100Ah is even enough to buffer through a full overcast UK winter day realistically, or if I'm kidding myself and need to look at a second battery too.

@OffGridGuru you've not finished your post mate — cuts off right after "feeding into"!

Watching this thread with interest though. Running a very similar setup on my narrowboat — Victron SmartSolar 100/20 feeding a pair of Fogstar Drift 100Ah lithiums, and honestly the Victron ecosystem just clicks once everything's talking to each other through the VictronConnect app.

One thing I'd say for a garden office specifically: think hard about your worst-case winter days. I learned that lesson the hard way on the cut — November on a canal with trees either side is basically a solar black hole. What's your battery bank looking like? That'll matter far more than the controller choice for keeping the lights on through a grey January fortnight.

VickyMurray | 847 posts | ⭐ Regular

@OffGridGuru Ha, no judgement here - I ran an extension lead to my summerhouse for three years before finally sorting it properly! 😄

I've got a Victron SmartSolar 100/20 running a 200W panel into a 100Ah lithium in my garden office and it's been absolutely brilliant. The Bluetooth monitoring through the VictronConnect app is genuinely useful rather than just a gimmick - I can see exactly what my kettle and monitors are pulling without going outside.

One thing I'd say - don't underestimate your loads. I thought I was being conservative with my estimates and still ended up wanting more capacity within six months. What are you planning to run in there? Knowing that would help with sizing advice. Also, are you going lithium or AGM? Makes a big difference to what controller settings you'd need.

Looks like we've got two incomplete posts in a row — is the forum playing up today? 😄

@OffGridGuru really curious what controller you ended up going with after that 200W panel. I'm currently sizing up something similar for my own garden office and genuinely torn on whether to go with the Victron SmartSolar 75/15 or jump straight to the 100/20 for a bit of headroom. Did you factor in any expansion plans when you sized yours?

Also — single panel on the roof, is that south-facing or did you have to compromise on orientation? I'm working with a mostly south-east aspect and wondering how much that eats into usable output during winter months.

PikeAndrea | 312 posts | Regular

Ha, looks like the forum's eating posts for breakfast today! @OffGridGuru and @VickyMurray both cut off mid-sentence — might be worth trying a different browser or clearing your cache if you're having trouble posting.

@OffGridGuru — genuinely curious what controller you've ended up with. I've got a Victron SmartSolar 100/20 running a couple of panels into a 100Ah lithium in my workshop and it's been rock solid for about eighteen months. The Bluetooth monitoring alone is worth it, being able to check state of charge from the house is surprisingly handy.

Once you get your full post up, happy to help with sizing questions. Key thing is knowing your battery bank capacity and what loads you're actually running in the office day-to-day.