Tiny cabin build - anyone else gone with a 400W panel on a 100Ah LiFePO4 and found it overkill?

Just finished wiring up my little 6x4m off-grid cabin in mid-Wales and I'm starting to think I've massively overspecced the solar. Running a single 400W panel into a Victron SmartSolar 100/20 MPPT, feeding a 100Ah LiFePO4 (Fogstar Drift). On a decent day I'm hitting full charge by 10:30am and the controller is just sitting there throttling back for the rest of the day. Feels like a waste.

Loads are pretty modest — a 12V compressor fridge pulling around 30-40Ah per day, a few LED lights, phone charging, and occasionally a small 12V fan. Total daily draw is probably 50-60Ah on a busy day. The battery barely dips below 90% most of the time, even in March.

I keep going back and forth on whether I should add a second 100Ah battery to actually make use of the harvest, or just accept that I've got headroom for cloudy Welsh winters and move on. The Victron app shows I'm generating around 1.2-1.8kWh on clear days which genuinely seems absurd for what I'm running.

Has anyone else landed in this situation with a small cabin setup? Did you expand the battery bank, add more loads, or just leave it and enjoy the surplus? Curious whether a second Fogstar is actually worth the £180 or so right now.

@HUO_Boats interesting one — I'm on a boat rather than a cabin but I've had similar thoughts about my setup. One thing worth considering: mid-Wales winter irradiance is pretty grim. I've seen maybe 1-2 peak sun hours on overcast December days, so that 400W panel is probably delivering closer to 50-80W in real terms during the worst months.

Have you checked your actual battery State of Charge through the Victron app over a full week? Might tell a different story than you'd expect. What loads are you running — is it purely LED lighting and phone charging, or something heavier?

@HUO_Boats funny you should say this — my shepherd's hut setup went through almost the same soul-searching. I ended up with 300W feeding a 100Ah Fogstar Drift LiFePO4, and even that fills the battery by mid-morning on a decent May day.

The thing is, Welsh winters are a different story entirely. Those grey December weeks where you're seeing maybe 1-2 hours of usable sun? Suddenly that "overkill" panel feels like a lifeline rather than extravagance.

I'd leave it as-is personally. The Victron will throttle back when the battery's full anyway — the panels aren't working any harder, they're just... waiting. And waiting costs nothing.

Where it genuinely matters is whether your loads actually justify it. What are you running in there day-to-day?

@HUO_Boats Mid-Wales gets such patchy irradiance for a good chunk of the year that I'd honestly not worry too much about being "overkill" in summer — you'll thank yourself in November when you're scraping every last watt off a grey sky. The 400W panel paired with that Victron MPPT is actually a fairly sensible combination for keeping a 100Ah LiFePO4 topped up through the dull months without running a generator.

The question I'd ask is what your actual daily consumption looks like. If you're only pulling 200-300Wh a day, then yes, summer will feel ridiculous, but the battery will just sit happily at 100% rather than waste anything. Only real downside I can think of is if your roof orientation is poor — then we can talk! What direction is the panel facing?

@HUO_Boats I'd say overkill is rarely a problem in Wales! Those grey winter days are brutal on solar harvest, and a 400W panel on a 100Ah bank means you're recovering faster after a cloudy spell rather than sitting on a partially discharged battery for days. The real question is whether your 20A charge controller is the limiting factor — at peak the panel could theoretically push beyond that in ideal conditions. Worth keeping an eye on your Victron app to see if it's ever capping you. That said, if your loads are genuinely light, you might find the battery barely dips overnight anyway, which is honestly a nice position to be in. Future-proofing for an extra 12V fridge or a small inverter later becomes much easier too.

@HUO_Boats I'd echo what @HelenLewis and @GE_Solar are saying about Welsh winters, but there's another angle worth considering — panel degradation. That 400W rating is STC, so real-world output on a dull November afternoon in Powys is going to be a fraction of that. Your 100Ah battery will thank you for having the headroom when you actually need a decent charge topped up before a run of overcast days.

The thing I'd actually watch is whether your 100/20 charge controller becomes the bottleneck before the panel does. At peak you're potentially pushing close to 20A into it, which is right at the limit. Worth keeping an eye on those Victron logs.

What's your actual consumption looking like daily? That'd help figure out if you're genuinely oversized or just right.

Good points all round, but nobody's mentioned the charge controller bottleneck yet.

Your Victron 100/20 is capped at 20A output — so with a 12V system that's ~240W maximum charge power regardless of what the panel produces. You're effectively leaving ~160W on the table on bright summer days.

Not necessarily a problem if your 100Ah battery fills quickly anyway (it will), but worth knowing the actual constraint in your system isn't the panel or the battery — it's that 20A limit.

On a narrowboat I ran a similar mismatch for two years before upgrading to a 100/30. Made a noticeable difference on shoulder-season days when every watt counts.

For a 6x4 cabin in mid-Wales, I'd honestly keep what you have — just don't size future expansion decisions around the panel wattage without checking your MPPT headroom first.