Last systems in service in South of France

Seen a few posts lately about big AC-coupled setups in sunnier climates and it got me thinking about how these systems translate to UK conditions — specifically on a boat where space and weight are always the enemy.

Been eyeing up AC coupling via a Victron Multiplus-II with a grid-tie inverter on AC-OUT1. The frequency shift stuff is clever in theory but I've read enough horror stories about it being sluggish to react when the batteries are near full on a decent solar day. Less of a problem in the south of France, more of a problem when you get an unexpected sunny week in August and you're sat at a marina.

My current setup is fairly modest — 400Ah of Fogstar Drift LiFePO4 cells, Victron MPPT direct to the bank, keeps things simple. But I'm curious whether anyone's running a proper AC-coupled arrangement on a narrowboat or sailboat in the UK and whether it's actually worth the complexity over just chucking more MPPT controllers in.

The 48kWh single-enclosure thing mentioned in other threads sounds impressive but that's basically impossible on most boats. Realistically I'd be looking at maybe 15-20kWh max before the weight becomes a problem.

Anyone running AC coupling here in the UK, particularly in a marine context? Curious what inverter you paired with what and whether the frequency shifting actually behaves itself in real conditions. And has anyone had issues with DNO regs or marina shore power complications when AC coupling? That bit always makes me nervous.

EdKelly | 847 posts

@BazBurns good point about translating those southern French setups to UK conditions — the irradiance difference is massive and people forget that. On a boat you've also got the added complication that your battery bank is working harder through winter simply because the solar isn't topping it up properly between cycles. I run a fairly modest lithium setup on my narrowboat and the key thing I found was right-sizing the battery capacity to cover genuine low-sun periods rather than assuming you'll get regular full charges like you would in the Med. AC coupling also adds complexity and inverter losses that frankly aren't worth it below a certain array size. What's your rough panel wattage and location — estuary, canal, coastal? Makes a big difference to what's realistic.

XO_Sparks | 1,203 posts

@BazBurns the AC-coupling efficiency penalty really bites in low-irradiance conditions — you're converting DC→AC→DC through the inverter-charger when the Victron Multiplus is absorbing, and that round-trip loss matters a lot less when you're pulling 6kWh/day in Montpellier versus 1.8kWh on a grey Solent morning.

For a boat specifically, I'd be looking at DC-coupled MPPT (Victron SmartSolar 100/30 or similar) over AC-coupled Fronius every time. Simpler topology, fewer conversion stages, and the MPPT keeps harvesting usably right down to 150W array output — which is exactly the kind of patchy generation you're managing October through March up here.

Weight's also genuinely different once you factor the Fronius inverter mounting hardware versus a compact MPPT rail-mounted beside your Cerbo.

ExFirefighter | 312 posts

@XO_Sparks makes a fair point on the AC-coupling losses. On my narrowboat I went DC-coupled with a Victron MPPT specifically because the UK's diffuse light conditions mean you're scraping every watt you can — adding an inverter/charger conversion stage in the chain just compounds the losses when irradiance is already marginal.

Worth asking: what battery chemistry are you planning? LiFePO4 handles partial state of charge far better than lead, which matters enormously on a boat where you're not always fully cycling. Fogstar do decent cells at sensible UK prices if you're self-building.

What's your typical daily load and panel capacity? Those two numbers would help narrow down whether AC or DC coupling actually makes sense for your specific setup rather than just going on what works in the Languedoc.

BatteryAlan | 2,156 posts

@ExFirefighter DC-coupling is the right call for marine — every watt counts when a grey November sky in Chichester Harbour is giving you the solar output of a birthday candle. Running a Victron SmartSolar direct into a Fogstar Drift LiFePO4 bank on my van conversion taught me that cutting out the inverter-charger loop in low-irradiance conditions isn't just efficient, it's practically mandatory — AC-coupling is brilliant when you've got Mediterranean surplus solar to throw away, less brilliant when you're rationing 2 hours of weak winter sun off the Solent.