Three Phase Hybrid Inverter Connected to the Grid, Generator and Battery

Been lurking here for a while and finally have a question I can't find a clear answer to anywhere.

I'm planning a fairly ambitious setup for my tiny house build — it's going to be grid-tied but with the ability to island off when needed (we get some genuinely horrible outages in our area). I've also got a diesel generator as backup.

Here's where it gets complicated: the site has a three-phase supply, which I wasn't expecting when I bought the land. Most of the hybrid inverter discussions I see on here are single-phase, so I'm a bit lost.

What I'm trying to achieve:

• Use grid as primary source, topping up a battery bank (probably Fogstar lithium cells)
• Seamlessly switch to battery/generator when grid drops
• Generator to charge batteries and run loads simultaneously
• Eventually add solar on top

A few specific questions:

1. Does a three-phase hybrid inverter like the Victron Quattro (or similar) handle generator input cleanly, or does it get fussy about generator Hz stability?
2. Is it worth converting to single-phase at the consumer unit given a tiny house won't have massive three-phase loads anyway?
3. Any UK installers who've actually done this kind of hybrid setup? Most quotes I've had seem to be from people who are just guessing.

The tiny house itself won't have enormous power demands — induction hob, heat pump water heater, and the usual. But I want to do this properly rather than bodge something together.

Any pointers from people who've actually got three-phase experience would be brilliant.

Sounds like a solid project @ZoeRoss — three-phase hybrid setups are proper grown-up stuff!

For the UK specifically, you'll want to look at Victron Quattro units (three of them configured in three-phase). They handle grid, genny and battery simultaneously without fussing about it.

Few things worth knowing early:

• G98/G99 compliance — your DNO will need notifying, G99 if over 3.68kW export
• Generator needs to be clean enough for the inverters to accept it — cheap gennys can cause headaches
• Fronius also do decent three-phase hybrids if budget allows

Running emergency backup at my cabin taught me the hard way — get your transfer switching logic sorted before anything else, otherwise grid/genny conflicts will cook something expensive.

What's your battery chemistry leaning towards? That'll shape a lot of the rest of the design.

Hey @ZoeRoss, welcome to posting! One thing worth flagging early on — if you're grid-tied, you'll need to make sure your inverter is G98 or G99 compliant (G99 if you're over 16A per phase), and you'll need to notify your DNO before connecting. Some hybrid inverters that look great on paper aren't actually certified for UK grid connection, so check that before falling in love with any particular unit.

Also worth thinking about your generator interlock logic — you'll want to make sure the inverter handles the transition cleanly and doesn't try to export to the generator, which some cheaper units can struggle with on three-phase setups.

What inverter brands are you considering? That'll help narrow things down considerably. Victron, Solis and Growatt all have three-phase options with decent UK support. 🙂

Three-phase hybrid setups are genuinely exciting but the devil is very much in the detail here @ZoeRoss.

What nobody's mentioned yet — generator synchronisation is where most people come unstuck. Your hybrid inverter needs to handle three separate AC sources (grid, genny, battery inverter output) without them fighting each other. Not all "three-phase hybrid" inverters actually do this gracefully.

Victron's Quattro stack with a three-phase MultiPlus-II configuration handles this brilliantly through VE.Bus — it'll properly arbitrate between sources. I've run a similar arrangement on my motorhome conversion (single-phase admittedly) and source-switching behaviour varies wildly between manufacturers.

Key question: which inverter are you actually looking at? Because some cheaper units marketed as "hybrid" will just brown-out or fault when the generator and grid phase angles don't align perfectly.

That single decision shapes everything else downstream.

Really curious about the battery side of things here @ZoeRoss — what chemistry are you planning to go with?

I've been down a rabbit hole recently looking at LiFePO4 options for a similar-ish setup and the three-phase compatibility question is genuinely complicated when it comes to battery communication protocols. Some inverters (Victron's three-phase Multi setups for example) have very specific requirements around BMS comms — CANbus vs RS485 can be a real headache depending on which batteries you're looking at.

Are you going with proprietary batteries like the Pylontech route, or building something more custom? That decision probably affects your inverter choice more than anything else. What brands are you actually considering for the inverter itself?