Adding a battery to your existing solar system in Perth

Many Perth households installed solar a few years ago and are now looking at a battery. Battery prices have come down, Synergy's Midday Saver plan rewards shifting use out of the evening, and the technology has improved.

Adding a battery to a system you already have is a different job from installing solar and a battery together. In WA there are two ways to do it, and the rules that came in on 1 May 2026 changed which one usually makes sense.

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The two ways to add a battery

Path 1: replace your inverter with a hybrid (DC coupled)

A hybrid inverter runs your panels and the battery from one box. Your existing panels connect to the new hybrid, and the battery connects to its battery port.

Why this is usually the default:

• The battery charges straight from the panels on the DC side while the inverter exports AC, so energy is converted once rather than twice.
• One inverter manages solar, battery and export, which keeps compliance, monitoring and warranty with one brand.
• With a battery attached, SAA Technical Requirements v1.9 §4.2 allows a panel array above the usual limit of about 133% of the inverter rating, up to the inverter maker's own maximum. That maximum varies by model, so check the one on your quote. It matters if you want to add panels now or later.
• DEBS export eligibility is set by inverter capacity, so a 5 kW hybrid keeps DEBS.

What it costs you: your current inverter is retired, and the hybrid is part of the quote. If your inverter is getting old, that money was coming anyway.

Path 2: keep your system and add an AC-coupled battery

An AC-coupled battery has its own inverter and connects at your switchboard. Your existing solar inverter keeps converting panel power to AC, and the battery charges from that AC and discharges back to it.

When it fits: you have a recent, compliant system you want to keep as it is, with no plans to expand it.

What to know before you choose it:

• Under WA's rules since 1 May 2026, adding a battery to an existing system is treated as an alteration. In practice, that brings whole-of-site compliance into play: the whole site has to meet the current requirements, including the parts already installed.
• To qualify for the WA Battery Scheme, a VPP or a state loan, an AC-coupled battery needs a gateway device on Synergy's Supported Solutions List that supports both your existing inverter and the new battery. Some same-brand pairings (Sungrow or Fronius) can share export and site compliance through one monitoring app instead.
• Without an SSL-listed gateway or a same-brand pairing, export is fixed at 1.5 kW, and there is no state rebate, no VPP and no state loan.
• In practice, third-party gateway devices have had trouble meeting the requirements and getting through testing.
• Any later addition to an AC-coupled system (more panels, another inverter, more battery) is another alteration. Expanding it later is hard.
• Energy goes panel to AC to battery and back to AC, so you lose a little more in conversion than with a hybrid.

Get both quoted. Ask your installer to price a hybrid replacement and an AC-coupled retrofit side by side, and to name the gateway or same-brand pairing if they propose AC coupling.

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What your existing system needs to support a battery

Single-phase homes (most Perth homes): check the battery's continuous output (kW) against the loads you want to run in the evening, not just its storage (kWh).

Three-phase homes: make sure the battery or hybrid supports three-phase, or that your installer explains which phase it will back up.

Age of the inverter: an inverter that is near the end of its life, or more than about seven years old, is a strong reason to go straight to a hybrid rather than building around it. Older inverters may also lack the communication protocols a modern battery or gateway needs.

Western Power: your installer handles the connection application and confirms what export limit applies after the battery goes in.

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How big a battery?

Size the battery to your evening and overnight use, the energy you currently buy from the grid after the sun goes down. Undersizing is the costly mistake: adding capacity later usually means a second installation, and on an AC-coupled system it is another alteration.

If your evenings are heavy, it is also worth asking about more panels at the same time. A bigger array with a battery often wins for evening-heavy homes, because the extra daytime generation fills the battery as well as covering the day. A hybrid makes that easier, for the reasons above.

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Cost of adding a battery in Perth

Installed prices move with the brand, the capacity and how much switchboard work the job needs, so compare like with like. Our home battery cost guide for Perth covers current ranges.

When you compare the two paths, look at the total:

• Hybrid path: hybrid inverter plus battery plus installation. No gateway needed.
• AC-coupled path: battery plus installation, plus the gateway device (or the same-brand pairing) needed to keep rebate, VPP and loan eligibility, plus any work the whole-of-site check turns up.

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Can you claim the WA Battery Scheme rebate on a retrofit?

Yes, as long as the setup is eligible. The WA Battery Scheme ($130/kWh, maximum $1,300 for 10kWh and above) applies whether the battery goes in with new solar or is added to an existing system. The battery must be:

• 2kWh or larger
• Installed with an inverter or gateway on Synergy's Supported Solutions List for battery storage. For an AC-coupled retrofit, that means a gateway device that supports both your existing inverter and the new battery
• Installed by an SAA-accredited installer
• At a Synergy-connected property

Tesla Powerwall 3 is on the Supported Solutions List but is not marked for the WA Battery Scheme, so it does not qualify for the state rebate.

The installer submits the rebate claim on your behalf, and your quote should show it applied.

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WA Battery Scheme eligibility and rebate amounts can change. Check current terms on Synergy's website before you buy. Rules on alterations and gateways as of October 2026.