What size solar battery do you actually need in Perth?

The honest answer to "what size battery should I get" is rarely "the biggest one in the quote." A battery that's too big costs more, earns less rebate per kWh, and spends half its capacity sitting empty. The right size is the one that covers your evening and overnight load on a normal day — and for most Perth homes, that lands in a fairly narrow range.

The short version

For a typical Perth household with rooftop solar, 10 to 13.5 kWh of usable capacity is the sweet spot. It's enough to carry most homes from sunset through to the next morning's sun, it sits inside the federal rebate's full-rate band, and it matches the cap on the WA Battery Scheme. Bigger can make sense for specific reasons — a large evening load, backup, an EV — but that's a deliberate choice, not a default.

Start with your evening load, not your total usage

The number that matters isn't how much power you use in a day. It's how much you use after the sun goes down. During daylight your solar covers the house directly; the battery's job is the evening and overnight stretch.

A rough way to estimate it: take your daily usage and assume roughly half to two-thirds happens after about 3pm — cooking, hot water, heating or cooling, lights, screens. For a home using 18–20 kWh a day, that's often 9–13 kWh of evening and overnight load. That's the gap a battery is trying to fill, and it's why 10–13.5 kWh covers most households comfortably.

If your bills are higher — ducted air-conditioning, a pool, electric hot water, a full house in the evenings — your evening load is larger, and a 13.5 kWh battery (or a stackable system you can extend later) starts to make sense.

Perth has plenty of solar to store

Perth's advantage is sunshine. At about 5 peak-sun-hours a day on average, a standard 6.6 kW rooftop system generates roughly 28 kWh on a clear day — comfortably more than most homes use. The limiter usually isn't generation; it's storage.

Without a battery, a typical solar home self-consumes only around 35% of what it generates — the rest is exported for a few cents. Add a battery sized to your evening load and that self-consumption jumps to around 75%. That shift — from exporting at the feed-in rate to avoiding grid power at roughly 32¢/kWh — is where most of a battery's savings come from. Oversizing past your evening load doesn't move that number much; there's only so much evening to cover.

Where the rebates draw the line

The rebates reward sizing sensibly, and it's worth knowing exactly where the lines fall:

• The federal STC rebate pays the full rate on 5–14 kWh, then tapers: 60% on capacity between 14 and 28 kWh, and just 15% above that. Every kWh past 14 earns a smaller rebate than the one before it.
• The WA Residential Battery Scheme ($130/kWh, capped at $1,300 for Synergy customers) only counts the first 10 kWh.

Both signals point the same way: 10 kWh captures the full WA rebate, and anything up to 14 kWh stays in the federal full-rate band. A 13.5 kWh battery is a popular choice precisely because it sits at that edge — maximum sensible capacity before the rebate efficiency drops.

When bigger is the right call

There are good reasons to go past 13.5 kWh:

• Backup through a long outage. To run the house for a full evening and into the next day during a blackout, more stored energy buys more hours.
• An EV you charge at home. Charging a car overnight is a large, regular evening load a standard battery can't cover on its own.
• A genuinely high evening baseline. Big homes running ducted climate control all evening can use a 13.5 kWh battery dry before morning.

In these cases a stackable battery is often the smart move — start at a sensible size and add modules later as your needs or budget grow, rather than paying for capacity you don't use yet.

When smaller is fine

If your evening load is modest — a smaller home, gas hot water and cooking, no air-conditioning — a battery below 10 kWh can be plenty. You'll still capture the WA rebate (it scales down with size) and cover your evenings. There's no prize for buying storage you won't cycle.

Nail your number

Sizing well is mostly about matching the battery to your evening load and staying inside the rebate bands. Two ways to get a personalised figure:

• Run your usage and goals through the Find My Battery flow — it sizes to your Synergy bill and shows the rebate you'd qualify for.
• Or line up specific models, capacities and warranties side by side on the comparison tool.

Sizing ranges are general guidance for a typical Perth home; your own evening load, solar size and goals decide the right number. Rebate bands use the federal STC schedule and the WA Residential Battery Scheme (first 10 kWh, $1,300 cap).