What Size Solar Battery Do I Need? Perth Sizing Guide 2026

The Quick Rule: Match Your Evening Usage

The simplest way to size a battery is to look at how much electricity you use between 5pm and midnight. This is the period when your solar panels aren't generating and you're drawing from the grid. A battery sized to cover this window delivers the best value.

For most Perth households, evening consumption sits between 8 and 12 kWh. A 10 kWh battery is the sweet spot — it covers roughly 70–80% of evening usage without oversizing.

Sizing by Household Type

Perth-Specific Sizing Factors

Perth has unique energy patterns that affect battery sizing:

• Air conditioning load: Perth's summers mean heavy AC usage from November to March. If you run a reverse-cycle AC in the evening, add 2–4 kWh to your evening consumption estimate. During a 40°C day, a single split system can draw 1–2 kW continuously.
• Long daylight hours: Perth averages 8–9 hours of strong solar generation daily, meaning a well-sized solar system can comfortably fill a battery by early afternoon. This is better than southern states — you can get away with a slightly larger battery relative to your solar system.
• Mild winters: Perth's mild winters mean relatively consistent battery cycling year-round. In Melbourne or Hobart, batteries often struggle to fully charge in winter — this is rarely a problem in Perth.
• Pool pump timing: If you have a pool, consider running the pump during solar hours (10am–3pm) instead of overnight. This reduces the load your battery needs to cover and can save 5–8 kWh of battery capacity.

How to Check Your Actual Usage

1. Synergy MyAccount: Log into your Synergy account and check your usage data. If you have a smart meter, you can see half-hourly consumption patterns showing exactly how much you use in the evening.
2. Electricity bill: Divide your quarterly kWh by 90 days to get your daily average. Then estimate that 40–50% of this is consumed between 5pm and midnight.
3. Solar inverter app: If you already have solar, your inverter app (Fronius, Sungrow, Huawei, etc.) shows how much you export during the day. This export amount is roughly how much a battery could store.

Common Sizing Mistakes

• Buying too small to save money: A 5 kWh battery for a 4-person home will fill by 7pm, leaving you on grid power for the rest of the evening. The cost per kWh is also higher for smaller batteries, reducing your value per dollar.
• Buying too large for future-proofing: A 20 kWh battery for a 2-person home will rarely cycle fully, meaning the bottom half of the battery sits unused. This wastes money and can actually reduce battery longevity (shallow cycling is less efficient than regular deep cycling).
• Ignoring your solar system size: Your battery can only charge from excess solar generation. A 6.6 kW system in Perth generates roughly 25–30 kWh per day, but after household daytime usage, you might have only 10–15 kWh of excess available to charge the battery. A 20 kWh battery paired with a 5 kW system will never fully charge from solar alone.
• Not accounting for degradation: Batteries lose capacity over time. A 10 kWh battery warranted to 70% retention will effectively be a 7 kWh battery at year 10. Consider sizing slightly above your needs to account for this.

The WA Rebate Sweet Spot

The WA Battery Scheme rebate applies to the first 10 kWh of capacity at $130/kWh (Synergy). This means a 10 kWh battery captures the full $1,300 rebate. Going larger than 10 kWh still makes sense if your household needs it, but the extra capacity above 10 kWh won't attract additional WA state rebate (federal STCs still apply to the full capacity).