How to Size Your Solar Battery for a Perth Home
The right battery holds what you use after the sun goes down, and no more than your solar can refill the next day. Here’s how to work that out, with a calculator that does the maths for you.
- 10–13.5 kWh: Typical size for a Perth home with 6.6 kW of solar
- First 14 kWh: Gets the full federal battery discount
- $1,300: WA rebate for Synergy customers at 10 kWh
Size your battery in four steps
The calculator above follows the same method we use at a site assessment. If you’d rather work it out on paper, here’s how.
Find your evening and overnight usage
This is the energy a battery replaces, so it matters more than your total usage. Your Synergy bill shows your average daily usage. If you have an advanced meter, the interval data in your Synergy account shows exactly when you use power.
No interval data? As a rule of thumb, households that are out during the day use 60–70% of their power after sunset. Busy homes with people in during the day use closer to 45%.
Battery size (usable kWh) ≈ evening + overnight kWh ÷ 0.9
Check your solar can fill it
A battery only helps if it gets charged. In Perth, each kW of panels makes about 4.4 kWh a day across the year (more in summer, roughly half that in mid-winter). Take away what your home uses during the day, and what’s left is what you can store.
A 6.6 kW system makes about 29 kWh a day. If you use 10 kWh during daylight, around 19 kWh is spare. If your spare solar is smaller than your evening usage, adding panels can be better value than a bigger battery.
Plan for the next five to ten years
A battery lasts ten years or more, so size it for the home you’ll have, not just the one you have now. The common changes that push usage up are an electric car, moving from gas to electric cooking or heating, a pool, and working from home.
An average commute adds about 5–7 kWh a day of EV charging. If you can charge from solar during the day, that load doesn’t need the battery at all. See our EV charger installs.
Match it to real batteries and rebates
Batteries come in set sizes, so round to the nearest one, comparing on usable capacity. Then check the rebate bands: the federal discount is paid in full on the first 14 kWh and the WA rebate caps at 10 kWh. For most homes, landing between 10 and 14 kWh gets the best value per kWh.
A worked example
A family of four in Perth’s northern suburbs, out at work and school on weekdays, with a 6.6 kW solar system.
| Average daily usage (from the Synergy bill) | 24 kWh |
| Share used after sunset (about 60%) | 14.4 kWh |
| Allow for charging losses (÷ 0.9) | 16.0 kWh |
| Solar made on an average day (6.6 kW × 4.4) | 29 kWh |
| Spare solar after daytime use (29 − 9.6) | 19.4 kWh |
| Right size for this home | 13.5–16 kWh |
Why not go straight to 16 kWh? A 13.5 kWh battery covers most of this family’s evenings and sits inside the full-rate federal discount band. The last 1–2 kWh a night would come from the grid, but capacity above 14 kWh costs more per kWh. It’s a close call, and exactly the kind we talk through at the assessment.
Which battery size suits which home?
A starting point only. Two homes with the same number of people can use very different amounts at night.
| Usable size | Usually suits | Solar to pair with | Rebate note |
|---|---|---|---|
| 5–8 kWh | One or two people, gas cooking and heating, light evening use | 5–6.6 kW | WA rebate paid per kWh, so it’s smaller |
| 10 kWh | Couples and small families who are out during the day | 6.6 kW | WA rebate reaches its $1,300 maximum |
| 13.5 kWh | Families of three to four with reverse-cycle air con in the evening, for example the Tesla Powerwall 3 | 6.6–10 kW | Inside the full-rate federal band |
| 16–20 kWh | Larger homes, ducted air con, or an EV charged overnight | 10–13 kW | Federal discount reduced above 14 kWh |
| 25 kWh+ | All-electric homes with high overnight use, a pool and an EV | 13 kW+ | Check the numbers carefully |
How 2026 rebates change the right size
Rebates don’t change how much energy you use at night, but they do change which size gives you the best value. Both schemes below can be combined.
Federal Cheaper Home Batteries
Around 30% off an eligible battery. Since 1 May 2026 the discount is paid in full on the first 14 kWh of usable capacity and at reduced rates above that. The rate also steps down on 1 January 2027, so the same battery will attract a smaller discount next year. Read our federal battery rebate guide.
WA Residential Battery Scheme
$130 per usable kWh for Synergy customers, up to $1,300 at 10 kWh ($3,800 for Horizon Power customers). You must join an approved virtual power plant, such as Synergy Battery Rewards. Eligible households can also apply for a no-interest loan.
New and upgraded systems in the Synergy network also need to meet WA’s technical standards introduced on 1 May 2026. We check your eligibility for every scheme and handle the paperwork as part of your quote.
Where the battery goes matters too
Size isn’t the only thing that decides how long a battery lasts. Heat and salt air both shorten its life, so placement is part of the design.
In coastal suburbs like Mindarie, Mullaloo and Quinns Rocks, we mount batteries in a garage or on a sheltered wall where possible and use marine-grade fixings outdoors. If you already have a system that’s showing corrosion, our repair and maintenance team can take a look, whoever installed it.
Battery size calculator
Set the sliders to match your home. We'll suggest a battery size and show an average Perth day, hour by hour, so you can see what the battery actually does.
Your Synergy bill shows this as average daily usage. No bill handy? Tap the amount closest to your two-monthly bill:
Existing or planned. Most Perth homes have 5–10 kW.
Suggested battery size
13.5 kWh usable
An average day with a 13.5 kWh battery
Above the line: where your power comes from. Below: where spare solar goes. The strip underneath shows how full the battery is.
- Solar, used directly
- From the battery
- From the grid
- Charging the battery
- Exported
Battery Sizing — Common Questions
For most Perth homes with a 6.6kW system, a battery with 10 to 13.5 kWh of usable capacity is the right fit. A 6.6kW system makes around 29 kWh on an average Perth day, and after daytime use there is usually 10 to 18 kWh spare to store. Go bigger only if your evening usage is high and your solar can refill it.
Usually 13.5 to 20 kWh. A 10kW system makes around 44 kWh on an average Perth day, so there is often 20 kWh or more spare after daytime use. Size the battery to your evening usage rather than to the solar system. A bigger solar system mainly means the battery fills more reliably, including in winter.
A 10 kWh battery suits couples and smaller families who use around 8 to 11 kWh between sunset and sunrise. It is also the size where the WA Residential Battery Scheme rebate reaches its $1,300 maximum for Synergy customers. Families with air conditioning in the evening or an EV usually need more.
It can, if the battery holds at least as much as you use between sunset and sunrise and your solar makes enough spare energy to refill it the next day. The calculator on this page shows how full your battery gets and when it runs out on an average Perth day.
Only if the system is installed with backup capability, and you will need to keep some charge in reserve for it. Many batteries only back up selected circuits such as lights, the fridge and the internet. Tell us if backup matters to you and we will design for it at the site assessment.
Total (or nominal) capacity is everything inside the battery. Usable capacity is the part you can actually draw on, and it is the number to size against. Always compare batteries on usable kWh.
Not always. An average Perth commute uses around 5 to 7 kWh of charge a day. If you can charge the car from solar during the day, you may not need extra battery capacity at all. If it only charges overnight (see our home EV charger installs), add that amount to your evening usage when you size the battery.
The federal Cheaper Home Batteries discount is paid at its full rate on the first 14 kWh of usable capacity and at reduced rates above that. The WA Residential Battery Scheme pays Synergy customers $130 per usable kWh up to $1,300 at 10 kWh. For most homes, a battery between 10 and 14 kWh gets the best value from both.
Some batteries are modular and can be expanded later, others cannot. If you expect your usage to grow, for example with an EV or a pool, tell us at the assessment so we recommend a battery that can grow with you.
Perth winters produce roughly half the solar of summer, so a battery that fills every day in summer may only partly fill in June and July. We size for your year-round average and show you what to expect in winter.
Yes. Joining an approved virtual power plant, such as Synergy Battery Rewards, is a condition of the WA Residential Battery Scheme rebate. You can still install a battery without joining, but you will not receive the WA rebate.
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