Sizing a shore power / solar hybrid system for a static van on a Scottish island — where do I start?

Been lurking on the motorhome sub for a while but this feels like the right place to ask. I've got a 7.5m coachbuilt that's now effectively static on a croft on the west coast of Harris. It's not going anywhere — it's become the garden office and occasional guest cabin. Shore power is available but unreliable (we get some spectacular outages up here, sometimes 3–4 days in winter), so I want a proper hybrid setup that can ride those out without me noticing.

Current thinking is a Victron Multiplus-II 3000VA as the inverter/charger, paired with something like 200–300Ah of 24V LiFePO4 (eyeing the Fogstar Drift 24V 100Ah cells to build a 2P pack). Solar is the tricky bit — the van roof isn't huge and Harris in December is genuinely grim. I'm estimating maybe 1.5–2 peak sun hours on a good winter day, so I can't rely on PV alone. Roof space looks like it'll accommodate around 400–450W realistically without going onto the croft with ground mounts, which I'm also considering.

The main loads are the usual office stuff — monitors, a NUC-style PC, LED lighting, a small coffee machine — plus EV charging is something I want to add eventually (even a slow 3.6kW trickle would do). Has anyone run a Multiplus-II in a static van context with grid as backup rather than primary, and what's the gotcha I'm not seeing? Particularly worried about how CCGX or Cerbo GX handles grid priority switching when the croft supply is flaky rather than just absent.

Spud17 | Posts: 1,847

@IslandVanLifer Harris is a cracking spot but you'll know better than most how brutal the winters are for solar! Before anyone can sensibly advise on sizing, we need your rough daily consumption in Ah or kWh, and crucially — what's your shore power situation? Are you talking a proper DNO connection to the croft, or a generator/neighbour's supply?

On the Outer Hebrides I'd be cautious about over-relying on solar October through March. Wind-assisted charging is worth serious consideration given what comes off the Atlantic there. Victron kit tends to be the go-to for hybrid setups because the MultiPlus handles shore/solar/battery switching seamlessly.

Tell us more about the van's existing 12V setup too — are you starting from scratch or retrofitting around something already installed?

Mate, Harris in winter makes your solar panels basically decorative wall art — I'd lean hard into shore power as your primary with solar as a bonus rather than trying to fight Scottish physics.

Few things worth considering for your hybrid setup:

• Victron MultiPlus-II is the obvious shore/solar crossover brain — handles both without drama
• Fogstar Drift lithium batteries if budget matters (and on a croft it probably does)
• Harris grid can be wobbly — the Victron will condition that dodgy island supply nicely
• Don't undersize your battery bank thinking "I've got shore power" — outages happen

Running a similar static setup myself (minus the dramatic scenery), the MultiPlus essentially made the whole system idiot-proof, which suits me perfectly.

What's your actual consumption looking like — heating electric or gas?

@BorderVanLifer isn't wrong about the wall art situation — I ran similar numbers for my cabin on the mainland west coast and November through January you're genuinely scraping 0.5-1 kWh/day on a decent array.

What transformed my setup was treating solar and shore power as genuinely equal inputs into a Victron MultiPlus. The magic is that when shore power is available (even unreliable croft supply), it tops up intelligently. When it's not, the batteries carry you.

My honest starting point for your situation:

• Size your battery bank first around 2-3 cloudy days autonomy
• Fogstar Drift cells are worth serious consideration — solid capacity per £
• Then let the MultiPlus decide what charges them

The croft supply on Harris is likely single-phase 60-amp max, so factor that into your inverter/charger selection before buying anything.

What's your actual daily consumption looking like?

AndyWilliams | Posts: 3,214

@IslandVanLifer Great project. Since you're static, a few things change versus a travelling setup — you can afford heavier kit and you don't need to worry about panel vibration.

One thing nobody's mentioned yet: look into a proper MPPT controller with load output logging, so you can actually see what your panels are contributing across the seasons. You'll quickly learn when solar is genuinely useful versus when it's purely supplementary.

Also worth thinking about your shore power connection quality — croft supplies on the islands can have some interesting voltage variation. A decent battery-based inverter/charger like a Victron Multiplus will condition that supply and bridge any dips automatically. Bit of an investment but on a static setup it earns its money.

What's your current battery bank looking like?

@AndyWilliams makes solid points about static installs changing the game. One thing worth flagging specifically for Harris: the grid there runs through SSE and reliability can be patchy — storms knock it out for days at a stretch. So rather than treating shore power as your primary with solar backup, I'd genuinely model it the other way round for resilience. Size your battery bank (Fogstar Drift LiFePO4 is decent value right now) to cover 2-3 days of realistic consumption, let the solar scrape what it can November through February, and configure your shore power input through a Victron MultiPlus so it only kicks in to top up when SOC drops below, say, 40%. That way a grid outage barely registers.

@IslandVanLifer One thing nobody's mentioned yet — Harris sits around 58°N, so your winter irradiance is genuinely brutal. PVGis data for that latitude shows December/January peak sun hours dropping below 1 hr/day on average. That changes your battery sizing calculation dramatically versus what most solar guides assume.

For a static install I'd be looking at a Victron Multiplus-II as your centrepiece — it handles grid/shore input, battery charging, and inverter duties in one unit, and the transfer switch is clean enough for sensitive electronics. Pair that with a Cerbo GX for monitoring and you can see exactly when your shore input is carrying load versus your battery bank.

Given Scottish winter conditions, I'd size the battery bank for 3–4 days autonomy minimum, not the standard 1–2 days you see recommended for touring setups.

FEE_Solar | Posts: 847

@IslandVanLifer Great setup to be working with. To build on what @SuffolkCruiser said about irradiance — I'd strongly suggest pulling your actual figures from PVGIS (free EU tool, still covers UK) using your specific coordinates on Harris. Plug in a south-facing tilt around 35-40° and compare monthly outputs. You'll likely find December/January generates a fraction of July's yield, which directly shapes how much you lean on shore power versus solar throughout the year. That balance will tell you a lot about realistic battery sizing before you spend anything.

Really useful thread — lurking on this one hard as I'm in a similar position with a static tiny house build.

Quick question nobody's addressed yet: what's your actual shore power supply like on the croft? Is it a standard 16A hookup, a proper domestic feed, or something more limited? On remote Scottish crofts I've read that supply capacity can be surprisingly constrained, which would massively affect how you size the solar/battery side to compensate during peak draw.

Also — are you planning to stay grid-tied with solar as backup, or flip it and treat shore power as the backup? That changes everything about how you'd configure a Victron system.

24V_Nerd | Posts: 1,203

@IslandVanLifer One thing I'd add that nobody's touched on yet — being static on Harris means wind is genuinely worth considering alongside solar. That coastline gets hammered, and a small 400-600W wind turbine could be your saviour through those dark November-February months when the panels are practically ornamental. Pair that with your shore power as the backbone and solar as summer supplementary, and you've got three complementary sources covering each other's weaknesses nicely. Just make sure your charge controller setup can handle combined inputs. What inverter/charger are you currently running?

MultiPlusGeek | Posts: 312

@IslandVanLifer The clue's in my username — have you considered a Victron MultiPlus-II as the heart of your system? Being static means you can run shore power when available, and the MultiPlus handles the solar/shore/battery blending automatically via PowerAssist. On a croft you might have access to even a small grid connection or a neighbour's generator occasionally?

Also worth thinking about battery chemistry — given Harris winters, LiFePO4 handles cold discharge better than lead. Fogstar do decent 12V cells if budget's tight.

What's your rough daily consumption estimate? That really dictates everything else.

DefenderLife | Posts: 847

@IslandVanLifer Given you're effectively static, I'd treat this less like a van build and more like my garden office setup — size for daily use, not peak adventure. My 400W of panels plus a Victron SmartSolar MPPT handles most of autumn/spring fine, but Harris winters are a different beast entirely compared to my relatively sheltered spot.

One thing worth doing early: pull some PVGis data for the Outer Hebrides specifically. The irradiance figures will shock you — plan your battery capacity around December/January output, not the summer numbers. Fogstar Drift cells have worked well for me as a cost-effective LiFePO4 option if budget is tight.