How to size a home battery for Perth: matching capacity to your household

When shopping for a solar battery in Perth, the most common question is: "How much capacity do I need?" The answer depends on three factors: your evening household consumption, how much solar surplus you generate during the day, and whether you want meaningful backup capability during grid outages.

The sizing principle: match to evening consumption

A battery's job (in most Perth households) is to store surplus solar generated during the day and discharge it during the evening — the Synergy peak rate window (3pm–9pm on Midday Saver: 55.33c/kWh) is when battery discharge is most financially valuable.

Evening consumption: Your evening load is what runs after solar generation ramps down (approximately 3–4pm in winter, 5–6pm in summer). In a typical Perth house:

• Lights, TV, computers: 0.3–0.8kW
• Cooking (electric oven/induction): 1–3kW for 30–60 minutes
• Air conditioning (summer evenings): 1–3kW
• Dishwasher: 1.2–2kW for 60–90 minutes

A typical Perth evening might draw 8–15kWh between 3pm and 11pm.

Sizing by consumption bracket

| Household | Evening consumption (3pm–11pm) | Recommended capacity | Common battery | |---|---|---|---| | 1–2 people, small home | 5–8kWh | 5–8kWh | BYD HVS 7.7kWh, Sigenergy 5kWh | | 3–4 people, average home | 8–15kWh | 10–13kWh | BYD HVM 11.04kWh, Sungrow SBH-10 | | 4–5 people, high consumption | 15–22kWh | 13–20kWh | BYD HVM 16.6kWh, Sigenergy 15kWh | | Large home or EV charging | 20kWh+ | 20kWh+ | Multi-unit stacked systems |

Most Perth households fall in the 10–13kWh range. The popular 10kWh options (BYD, Sungrow, Sigenergy) match this well.

Does your solar system generate enough to fill it?

A battery only helps if your solar system generates enough surplus to fill it during the day. Oversizing the battery relative to your solar surplus means the battery frequently charges only partially — wasting the capacity you paid for.

Rule of thumb: Your solar system should generate approximately 1.5–2× the battery capacity in surplus per day during summer to reliably fill the battery.

Example:

• 6.6kW system on Midday Saver, summer
• Household daytime consumption: 10kWh
• Solar generation: 28kWh/day
• Surplus available to charge battery: ~18kWh/day
• A 10kWh battery fills comfortably from this surplus

In winter, the same system might generate only 12kWh/day with 6kWh surplus — barely enough to partially fill a 10kWh battery. This is a structural limitation of batteries in Perth winter (see our winter solar performance guide).

Do you want backup?

If backup capability during grid outages matters, sizing should account for how long you want backup, not just evening discharge.

During a daytime outage: Solar can potentially power the house directly if the battery and inverter support backup/island mode. Not all battery + inverter combinations support this.

During a night outage: The battery is the only source. 10kWh covers an average Perth house for approximately 8–12 hours at moderate consumption. Essential loads (fridge, lights, phone charging): 2–3kWh overnight, giving 30–40+ hours of essential backup from 10kWh.

If backup is important:

• Confirm the battery + inverter combination explicitly supports backup/island mode (not all do)
• Size to cover essentials overnight: 3–4kWh minimum; 7–10kWh for comfortable coverage
• Larger systems (13–16kWh) give multi-day essential backup

The WA Battery Scheme rebate covers up to 10kWh battery capacity at $130/kWh. Systems above 10kWh receive the maximum $1,300 rebate — the extra capacity above 10kWh is at your cost with no additional rebate.

Common sizing mistakes

Undersizing (3–5kWh battery for a large household): The battery fills and stops accepting charge while surplus solar continues to be exported at DEBS rates (2c/kWh off-peak). You're losing the opportunity to store and discharge at 55.33c/kWh evening value. Economic return is poor.

Oversizing (16–20kWh battery for a small household): The battery never fully charges from available solar surplus. Most of the capacity is unused. High upfront cost for capacity that doesn't deliver proportionate return.

Ignoring winter in sizing: A battery sized for summer surplus (18kWh available) may rarely fill in winter (6kWh available). If winter bill reduction is a priority, don't oversize the battery based on summer generation only.

A practical sizing approach

1. Get 3 months of Synergy bill data — look at daily consumption in summer, shoulder, and winter
2. Identify your evening consumption — check your bill's interval data in MyAccount, or upload your bill to BillWise for analysis
3. Match battery capacity to evening consumption, not total daily consumption
4. Confirm your solar surplus — if you already have solar, your inverter monitoring app shows daily generation vs consumption
5. Account for 20% charging losses — a 10kWh battery stores approximately 10kWh but charging and inverter losses mean you need approximately 11–12kWh of solar surplus to fill it

Battery capacity needed varies significantly by household. The BillWise calculator can help estimate the right size based on your specific Synergy bill data.