Typical daily usage for a Perth home
Share of power used after sunset
Usable capacity most families land on
The four-step sizing method
1. Start with daily kWh, not dollars
Your electricity bill lists average daily usage in kWh. A typical Perth home sits between 15 and 25 kWh a day; larger homes with ducted air-conditioning and a pool push past 35 kWh. This number sets the ceiling for every sizing decision that follows.
2. Split usage into daytime and evening
Solar covers daytime load directly, so a battery only needs to carry what you consume after sunset. Most households use 40–60% of their power in the evening and overnight. For a 20 kWh home, that is roughly 8–12 kWh of night-time load.
3. Check how much solar you're exporting
A battery can only store surplus. If your system exports 10 kWh a day, storage above that just sits empty. When export is small, adding panels before storage almost always pays back faster.
4. Buy usable capacity, and leave headroom
Quotes should state usable kWh, not nameplate. Size to cover your evening load with 10–20% spare so the battery is not cycled to empty every night — depth of discharge is what wears cells out, and warranties are written in cycles.
Sizing table for WA homes
Indicative starting points. Your final design depends on roof orientation, tariff and whether you want blackout backup.
| Household | Usage | Solar | Battery |
|---|---|---|---|
| 1–2 people, no A/C | 10–14 kWh/day | 6.6 kW | 8–10 kWh usable |
| Family of 4, split A/C | 15–22 kWh/day | 6.6–10 kW | 10–13 kWh usable |
| Family + ducted A/C or pool | 23–32 kWh/day | 10–13.2 kW | 13–20 kWh usable |
| Large home + EV charging | 33 kWh+/day | 13.2–15 kW | 20–30 kWh usable |
Battery sizing FAQs
Is a bigger battery always better?
No. A battery only earns money on the energy it cycles. If your evening load is 10 kWh, a 25 kWh battery spends most of its life half full — you paid for capacity that never offsets a bill.
Do I need enough capacity to run through a blackout?
Backup sizing is a separate calculation. For essential circuits — fridge, lights, internet, a few power points — 5–8 kWh of reserve is usually plenty. Whole-home backup with air-conditioning needs far more, plus a backup-capable inverter.
Can I add more capacity later?
Most batteries we install are modular, so you can stack extra kWh onto the same inverter later. Tell us at design stage and we will leave the headroom and cabling for it.
Does battery size affect the WA rebate?
The WA Residential Battery Rebate is a set amount per eligible property rather than per kWh, so a bigger battery does not increase it. Federal STCs scale with solar system size, not storage.
Want the numbers run on your actual bill?
Send us a recent bill and we'll model usable capacity, savings and rebate eligibility for your home.