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The right size depends on how much electricity your home uses after the sun goes down, because that is the load a battery actually covers. Two fixed rules on the Synergy network give you firm anchors. A battery needs at least 5 kWh of storage to qualify for the WA Residential Battery Scheme rebate, and the rebate stops adding value past 10 kWh because it pays $130 per kWh of capacity, capped at $1,300. For most Synergy households the practical question is not "how big can I go" but where in the 5–10 kWh band your evening usage lands.
This is general guidance for sizing, not advice on a specific quote. Your installer sizes the system to your roof, your usage and your budget. Rebate and tariff details change, so confirm the current rules on Energy Policy WA and Synergy's official pages before you commit.
The WA Residential Battery Scheme rebate is tied directly to battery capacity, so size and rebate value move together up to a ceiling. For Synergy customers the state rebate is $130 per kWh, to a maximum of $1,300. Do the arithmetic and the cap lands at 10 kWh. Every kWh beyond that earns no extra state rebate.
The WA rebate stacks with the federal Cheaper Home Batteries Program, which is delivered through small-scale technology certificates (STCs). For a 10 kWh battery installed from 1 May 2026 the federal support is worth about $2,700, so combined support comes to roughly $4,000. The federal certificate rate steps down at half-yearly intervals, so the exact amount depends on your installation date. Keep the figures separate when you read a quote: $1,300 is the WA state rebate and the rest is federal, not a single scheme's payment.
There are also no-interest loans of up to $10,000 for households with a combined annual income under $210,000, repayable over up to 10 years. The loans can cover batteries as well as new or upgraded inverters and panels installed with a battery.
The takeaway for sizing: the rebate structure rewards capacity up to 10 kWh and nothing above it. A larger battery can still make sense for your home, but it does so on its own merits, not on extra rebate.
A battery system needs a minimum of 5 kWh of storage capacity to be eligible under the WA Residential Battery Scheme. New, additional and replacement systems can all qualify.
Synergy's Battery Rewards VPP program has its own technical thresholds. Batteries, alone or combined, must meet Synergy's minimum "5 unit" storage requirement, and inverters need a combined capacity of at least 2.5 kVA. The system also has to appear under the DER-Storage category on Synergy's Supported Solutions List and meet Synergy's functionality requirements.
Inverter capacity is the other half of sizing, and the rules changed recently. From 1 July 2025 the WA Government, working with Western Power, moved to allow larger single-phase inverters of up to 10 kVA to connect to the network. For Battery Rewards participation the floor is a combined inverter capacity of at least 2.5 kVA.
In practice the inverter sets how fast the battery can charge and discharge, and the network sets how large that inverter can be. Ask your installer to confirm the inverter rating on your quote against the current connection limits for your property.
Start with the load a battery covers: the electricity you draw when your panels are not producing, through the evening, overnight and early morning. A battery is sized to store enough daytime solar to carry that load, not to match your total daily consumption.
The figure that drives this is your evening and overnight kWh, and it varies widely between Perth homes. Synergy notes that average daily consumption "can vary greatly from household to household" depending on occupants, habits and lifestyle, and points customers to the usage history in My Account. Pull your own interval data from My Account before you settle on a size. It is the single most useful number you can bring to a quote.
Your tariff matters too, because it changes whether storing a kWh beats exporting it. On the standard A1 residential tariff Synergy charges a flat 33.2621 cents per kWh from 1 July 2026. Every kWh your battery lets you self-consume in the evening is a kWh you don't buy at that rate. What Synergy pays you to export instead of store is set by the DEBS buyback rate, so check the current rate on Synergy's website when you compare quotes.
From 1 May 2026, all new and upgraded rooftop solar and battery systems in the South West Interconnected System (SWIS) must comply with minimum technical standards set out in Western Power's WEM Procedure for Standard Small User Facilities. Synergy states these are designed to enable larger compliant solar and battery systems and broader participation in VPPs. If you're sizing a system now, ask your installer to confirm it meets these standards, since they sit on top of the battery scheme's own eligibility rules.