Adding a battery to your existing solar system in Perth

Many Perth households installed solar 3–7 years ago and are now considering adding a battery. Battery prices have fallen significantly, Midday Saver tariff arbitrage makes batteries more financially compelling than they were previously, and battery technology has improved.

The process of adding a battery to an existing system differs from installing solar and battery together from scratch.

How battery retrofit works: AC-coupled vs DC-coupled

There are two ways to add a battery to an existing solar system:

AC-coupled (most common for retrofits)

An AC-coupled battery connects to your home on the AC side of the existing solar inverter, not to the solar panels directly. It can charge from solar via the home's AC circuits, from the grid (when rates are low), or both.

How the energy flows:

• Solar generates DC power → existing inverter converts to AC → some AC powers home loads → surplus AC charges the battery (via a built-in battery inverter/charger) → battery stores as DC
• When you need power from the battery: battery DC → battery inverter converts to AC → powers home loads

Pros of AC-coupling:

• Works with virtually any existing solar inverter brand
• No changes to existing DC wiring or solar inverter
• Simple installation — one new box (the battery) connects to your switchboard
• Battery can also charge from the grid (useful for Midday Saver: charge at 8.85c/kWh, discharge at evening peak rates)

Cons:

• Energy is converted DC→AC→DC→AC twice rather than once, with minor efficiency losses (~5–10%)
• The battery's built-in inverter/charger needs to be sized correctly

Compatible AC-coupled batteries: Tesla Powerwall 2, BYD Battery-Box Premium (with separate battery inverter), Enphase IQ Battery 5P/10T, Alpha-ESS Storion SMILE series, Sungrow SBR (with compatible inverter).

DC-coupled (replaces or adds to the inverter)

A DC-coupled retrofit connects the battery to the solar panels on the DC side, sharing the solar inverter function via a hybrid inverter.

What this requires: Replacing your existing solar inverter with a hybrid inverter that manages both solar input and battery. The existing panels connect to the new hybrid inverter's DC inputs.

Pros:

• More efficient (DC→AC conversion happens once, not twice)
• Tight integration between solar and battery management

Cons:

• Requires replacing your working solar inverter — a significant cost ($1,500–$3,000 for the inverter itself)
• Only makes sense if your existing inverter is near end-of-life or needs replacement anyway
• Panel compatibility with the new hybrid inverter needs to be confirmed

DC-coupled retrofit is usually not cost-effective unless you're replacing the inverter anyway. For most Perth households, AC-coupled is the right retrofit choice.

What your existing system needs to support a battery

Single-phase homes (most Perth residential): Nearly all AC-coupled batteries work. Check the battery's maximum continuous discharge rate (kW) against your expected peak loads — for most Perth homes, 5kW discharge capacity is sufficient.

Three-phase homes: Ensure the battery supports three-phase or that your installer properly phases the battery connection. Some single-phase batteries can be used in three-phase homes with modifications.

Export limit: Adding a battery doesn't change your existing Western Power export limit. If you're in a 1.5kW constrained suburb, your existing limit remains. The battery simply absorbs generation that would otherwise be curtailed, which is the primary financial benefit in constrained suburbs.

Age of solar system: Very old solar inverters (10+ years) may have firmware or communication protocol incompatibilities with modern battery management systems. Your installer should check this. In some cases, an older inverter may not communicate current export limits to the battery correctly.

Cost of battery retrofit in Perth

AC-coupled battery retrofit (supply + install):

| Battery | Usable capacity | Installed cost range | |---|---|---| | Tesla Powerwall 2 | 13.5kWh | $13,500–$15,500 | | BYD Battery-Box Premium HVS 10.2 | 10.2kWh | $10,500–$12,500 | | Enphase IQ Battery 10T | 10.08kWh | $11,000–$13,500 | | Alpha-ESS Storion SMILE 10 | 10.1kWh | $9,500–$11,500 | | Sungrow SBR 10kWh (AC-coupled) | 9.6kWh | $9,000–$11,000 |

After WA Battery Scheme rebate ($1,300): subtract from above costs.

Retrofit additional costs compared to greenfield installation:

• AC coupling switchboard work: usually included in the above
• If communication setup between battery and inverter requires additional hardware (some brands): $200–$500

The cost is broadly similar to a greenfield battery-only installation — there's minimal retrofit premium since AC-coupled installation doesn't touch the existing solar hardware.

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

Yes. The WA Battery Scheme ($130/kWh, maximum $1,300 for 10kWh and above) applies to battery installations regardless of whether they're installed alongside new solar or retrofitted to an existing system. The battery must be:

• 2kWh or larger
• On the Synergy SSL (Scheme-subsidised list)
• Installed by a CEC-accredited installer
• Applied to a Synergy-connected property

The installer submits the rebate claim on your behalf — the battery purchase price in your quote should already reflect the rebate applied.

Does adding a battery require a new Western Power application?

In most cases, no — adding an AC-coupled battery to an existing connected solar system does not require a new Western Power application. The connection is already approved. However:

• If you're in a zero-export zone and the battery needs export capability, confirm with your installer
• If the battery's inverter/charger rating exceeds certain thresholds, there may be notification requirements
• Your installer should confirm the specific Western Power requirements for your situation

WA Battery Scheme eligibility and rebate amounts are subject to annual change. Verify current terms at Synergy's website before purchasing. Battery prices and compatibility information are as of mid-2026.