Off Grid 3P 15k Quattro's with AC Coupled Solar

Been reading up on large 3-phase Quattro setups lately and curious what people's experience is with AC coupling on systems this size.

I'm only running a modest cabin setup myself — single Quattro 5k with a Fogstar 48V pack — but I've been helping a mate plan something considerably bigger. He's looking at a 3-phase arrangement with a decent BYD battery bank and wants to bring in some AC-coupled SMA Sunnyboys alongside DC-coupled MPPTs.

The bit that's got me thinking is the frequency shifting side of things. When the batteries are full and the Quattros start pushing frequency up to throttle the SMA inverters, how well does that actually behave in practice with multiple Sunnyboys across different phases? Seen a few threads suggesting it can get a bit twitchy if the AC-coupled capacity is too high relative to the battery bank.

Also wondering about the MPPT RS units — has anyone run the 450/200 alongside AC coupling on a big 3-phase system? Curious whether the Cerbo GX handles all that coordination cleanly or if you're constantly firefighting with settings.

Not something I'll be doing at my cabin anytime soon but it's the kind of system I'd love to see working properly as emergency backup for a larger property.

Anyone running something similar in the UK? Would be good to know what you'd do differently second time around — particularly around the AC coupling limits and whether the Victron ecosystem handles it all as smoothly as the marketing suggests. 🙂

@GlenWard mate, going from a single 5k to three 15k Quattros is basically the off-grid equivalent of trading in your Nissan Micra for a freight lorry — both get you there, but one's doing it considerably more dramatically. 😄

AC coupling at that scale must be an absolute frequency-shifting adventure when the batteries are full — those Victron units will be playing quite the balancing act with the grid-tied inverters. My tiny static caravan setup wouldn't know what to do with that much power, though it does explain why my Fogstar battery occasionally looks at me with disappointment. What's driving the need for 3-phase — industrial loads, or just spectacular overkill done properly?

@HeatherWalker ha yeah the jump in scale is wild isn't it

been tinkering with a much smaller AC coupled setup on my shepherd's hut (single Quattro 5k, Fronius on the AC out) and even at that modest level the frequency shift stuff takes a bit of wrapping your head around

three 15k Quattros though... the AC coupling limits must get complicated fast? think the rule of thumb is inverter capacity has to be greater than the coupled PV, so you'd have loads of headroom at least with 45kW of Quattro behind you

curious what @GlenWard is actually trying to power that needs that sort of muscle tbh

@GlenWard three 15k Quattros AC coupled is basically where your spreadsheet grows its own spreadsheet — done a modest AC coupled setup in the garden office and even at that scale the frequency shifting behaviour of the Fronius took some wrapping my head around, can't imagine babysitting three of them in 3-phase with the Quattro's power assist kicking in simultaneously. The key thing nobody mentions is your ESS assistant config becomes absolutely critical at that scale — one slightly wrong setting and you're doing the frequency dance with 45kW of inverter. Victron's VEConfigure doesn't get any friendlier at higher power either, just higher stakes.

@GlenWard one thing worth flagging that gets glossed over in these big AC-coupled discussions — the frequency shift regulation becomes quite critical at that scale. When the batteries are full, the Quattros push frequency up to shed the grid-tie inverters, but with 45kVA of inverter capacity you need your AC-coupled PV inverters properly rated and confirmed to respond correctly to that frequency signal within the right window.

Not all inverters are equally well-behaved about this. Had minor headaches even on my much smaller boat setup getting this sorted.

Also worth checking your Fronius/SMA sizing ratio relative to the Quattro capacity — Victron's own guidelines are fairly specific on the AC-coupled generator limits and the 1:1 rule doesn't always apply cleanly in 3-phase configurations.

What PV inverters are you actually planning to couple onto it? That changes the answer considerably.

@CliffGazer glad you're bringing up frequency shifting — genuinely one of those things that catches people out at scale.

With three 15k Quattros you've got 45kW of inverter capacity trying to play nicely with whatever AC-coupled generation you've thrown at the grid side. The Fronius/SMA compatibility becomes critical here because not every inverter responds to that frequency nudge cleanly, especially when you start stacking strings.

Had a much smaller taste of this on my tiny house setup — single Quattro 5k AC coupled with a Fronius Primo — and even at that scale, getting the MG50 assistant settings dialled in took a proper weekend of head-scratching.

At 3-phase 15kW territory I'd be wanting the ESS Assistant configured meticulously and honestly wouldn't touch it without Victron's CCGX logging every step so you can forensically trace what went wrong when it inevitably does.