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A 15kW three-phase inverter does not give you 15kW on any one phase. It gives you roughly 5kW per phase. If your ducted air conditioner is a single-phase unit drawing 6kW on startup, and it sits on one of those phases, the battery cannot carry it on its own. During daylight the solar and the grid make up the difference and nobody notices. After sunset, running on battery, the inverter trips or the aircon drops out.
That is the whole mechanism. It is not a fault, and it is not something a firmware update fixes. It is the arithmetic of how three-phase inverters distribute their output, and it needs to be settled at design stage.
Evening loads during an outage
During the day there are three sources feeding your loads: the solar, the battery, and the grid. The aircon draws what it needs and the shortfall on that phase comes from wherever it can.
After dark the solar is gone. If you are in a blackout, the grid is gone too, and the battery is on its own against a load it was never sized to carry on a single phase. The compressor tries to start, the phase hits its ceiling, and something gives.
We have had this conversation on the phone more than once with people who bought a large three-phase system specifically so the house would ride through a summer outage, and then found out in January that the ducted system was the one thing it would not run. They were not sold a faulty product. They were sold a system nobody had sized against their actual evening load.
The four backup configurations
Backup is not a yes or no field on a quote. There are four distinct arrangements, and the difference between them is the difference between a system that works the way you pictured and one that does not.
Whole home backup. Everything in the house runs during an outage. This requires the battery and inverter to handle your full simultaneous load, per phase, with no help from the grid. It is achievable, but it usually costs more than people expect and it needs honest load figures to design against.
Partial backup, downstream circuits. A selected set of essential circuits sits behind the battery. Lights, fridge, power points, maybe one split system. When the grid is up, those circuits can draw the full available capacity. When it is down, only the backed-up circuits live. This is the most common arrangement and, for most homes, the sensible one.
Partial backup, upstream circuits. Certain heavy loads are deliberately excluded from the battery entirely. EV chargers are the usual candidate, sometimes the ducted aircon. The exclusion is a design decision, made so the rest of the system behaves predictably.
No backup. The battery does energy shifting only. In an outage it shuts down with everything else, because AS/NZS 4777.1 anti-islanding requires it to disconnect unless the system is specifically built to island.
Most quotes say "battery backup" and mean the second option. Most customers hear the first.
Pass-through current per phase
There is a second number that matters and it rarely makes it onto a quote. When the grid is connected, current passes through the inverter to your loads, and the inverter has a limit on how much.
Three-phase units vary widely here. Some are rated at 32A per phase, others at 80A. That difference decides whether the system can sit in line with your whole switchboard or whether it has to be wired to a subset of circuits. It has nothing to do with the kWh on the front of the brochure, and two batteries with identical storage can behave completely differently on this point.
If you are being quoted a three-phase battery, ask for the continuous pass-through current per phase. If the answer is vague, that is information too.
Ducted aircon and backup sizing
Perth is a hard case for this, for two reasons that compound.
Ducted reverse-cycle is close to standard in newer builds here, and it is often a single-phase unit even in a three-phase house. And three-phase supply itself is common across the newer northern and southern corridors, so a lot of homes have exactly the configuration that produces this problem.
The practical version: a three-bedroom home on single phase with a couple of split systems is straightforward to back up. A larger home on three phase with ducted running into the evening is the one that needs real load figures before anyone quotes a backup outcome.
Our position, and it is one that costs us margin sometimes: if your goal is riding out summer outages with the ducted system running, either the system gets sized properly for that, with the per-phase capacity to do it, or you accept partial backup and put the aircon outside it. Selling a mid-sized three-phase battery on an implied promise of whole-home backup is where this goes wrong, and the customer does not find out until the first hot night with the street down.
Sometimes the right answer is a larger inverter with the per-phase headroom to carry the load. Sometimes the right answer is to spend less, take essential-circuit backup, and stop paying for a capability the house will not deliver.
Backup details to confirm
Ask these, and get the answers in writing:
Whole-home backup claims
Under Australian Consumer Law, what a salesperson tells you forms part of the agreement. A verbal assurance that the system would run the whole house during an outage is enforceable, and the remedy is a system that does what was promised or a refund. It is not a workaround, and it is not switching the battery off at night to stop the tripping.
If that describes your situation, get the original quote and any written or messaged claims together before you go back to the retailer.
For anyone still at the quoting stage, the five questions above will tell you more about a proposal than the headline kWh figure will. Get a quote that answers them.