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AC coupling used to be the comfortable way to add a battery to solar you already have: keep your panels, keep your inverter, bolt a battery alongside. The sales pitch still says that. In Western Australia, the rules no longer really do. We don't sell batteries, so here is the unvarnished version before you sign anything.
Quick answer
DC-coupled batteries store solar before it is converted, so a single conversion step delivers about 95% round-trip efficiency, and they suit new solar-plus-battery installations. AC-coupled batteries connect after your existing solar inverter at about 90% round-trip efficiency. But in WA the bigger difference is now regulatory: without an approved gateway or same-brand pairing, an AC-coupled system is export-limited and ineligible for the state scheme, and any later addition triggers whole-of-site compliance.
Replace or keep: the real decision
The question is no longer "how do I add a battery to my solar?" It's "is my existing inverter worth building around?"
Swap the old inverter for a hybrid and connect the battery to it. The whole site meets current requirements in one clean pass.
Often makes sense if:
Leave your solar inverter alone and add a battery with its own inverter.
Can still make sense if:
Go in knowing: the expansion constraint is permanent. Any later addition means bringing the whole site up to current requirements.
AC vs DC coupling, in one minute
Solar panels generate DC (direct current) electricity. Your home uses AC (alternating current). Every conversion between the two loses a little energy.
The appeal of AC coupling was always retrofit convenience: nothing about your existing solar had to change. That convenience is exactly what WA's current rules put under pressure.
| Feature | DC-coupled (hybrid) | AC-coupled |
|---|---|---|
| Hardware | ||
| Energy conversions | Works in your favour: 1 (DC → AC) | Watch out: 3 (DC → AC → DC → AC) |
| Round-trip efficiency | Works in your favour: ~95% | Watch out: ~90% |
| Inverters | Works in your favour: 1 hybrid handles everything | Watch out: 2 to maintain (solar and battery) |
| Use your existing inverter? | Watch out: No, needs a hybrid inverter | Works in your favour: Yes, if the site meets the WA gateway or shared-compliance rules |
| Battery compatibility | Watch out: Battery must be compatible with the specific inverter | Depends: Not tied to your solar inverter; the WA pathway still applies |
| Independence | Watch out: If the hybrid inverter fails, solar and battery are both affected | Works in your favour: Solar and battery systems operate independently |
| WA rules | ||
| Compliance at install | Works in your favour: A new hybrid meets today's rules in one pass | Depends: Needs an SSL gateway or same-brand shared compliance |
| Without an approved pathway | Works in your favour: Not applicable | Watch out: Export capped at 1.5 kW; no state rebate, VPP or loan |
| Adding more later | Works in your favour: Expand without dragging an old inverter through a compliance upgrade | Watch out: Any addition triggers whole-of-site compliance |
| Backup power | Depends: Depends on the full system design | Depends: Depends on the full system design, with more limitations |
What changed in WA
Rule 1
Under the compliance framework that took effect with WA's solar and battery reforms on 1 May 2026, any addition to an AC-coupled system counts as an alteration that triggers whole-of-site compliance. The entire site must then meet the current requirements, not just the new equipment.
Rule 2
A gateway device on Synergy's Supported Solutions List coordinates your existing solar inverter and the new battery so the site can meet export and control requirements.
With an approved gateway or same-brand pairing
Formally eligible for the WA scheme, VPP participation and the interest-free loan, subject to the gateway caveats below.
Without an approved pathway
1.5 kW export limit
A serious haircut on feed-in earnings, and ineligible for the WA scheme, VPP participation and the interest-free loan.
In practice
For every new or upgraded system
From 1 May 2026, new or upgraded solar and battery systems on WA's main grid (the SWIS: Perth and the South West) must meet Western Power's updated technical standards, set out in the WEM Procedure "Technical Requirements for Standard Small User Facilities." They apply to connections under 1,000 V with total inverter capacity of 30 kVA or less. (source: Energy Policy WA)
The efficiency difference is real but relatively small. For most households the deciding factors in WA are now regulatory (the eligibility pathway and the expansion rule) rather than the few percent of round-trip efficiency.
Whether a system provides backup power during a blackout depends on the complete system design, not on the coupling type alone. Both AC and DC coupling can support backup power, but the implementation differs and AC-coupled setups have more limitations. Discuss your backup requirements specifically with your installer.
"How does this site meet whole-of-site compliance?"
If the quote proposes AC coupling, the pathway (gateway or shared compliance) should be named on the quote, not assumed.
"What happens to my export limit?"
Without an approved pathway it's fixed at 1.5 kW. Ask for the number your setup lands on.
"What will adding capacity in five years actually involve?"
For an AC-coupled site, the honest answer includes whole-of-site compliance. A good installer will say so unprompted.
"How does backup power work with this configuration?"
If backup matters to you, understand the specific implementation.
For the full side-by-side on costs, which battery brands use each type, and the WA retrofit path, compare AC and DC coupled batteries.
Our matcher weighs coupling as part of the fit. Tell it you have existing solar and it will show you which batteries suit a keep path and which suit a replace path, rebate eligibility included.
Get a quote from an installer from our network that covers your area. You choose whether to act on it. No obligation.
AS/NZS 5139 clearance rules, garage vs outdoor walls, and what installers check on-site.
A step-by-step method covering evening usage and seasonal AC demand.
Activation events credit 70¢/kWh on top of DEBS exports, and signing up is a WA rebate condition.
Answer a few questions and see systems and installers that fit your home.