Smaller
About half your evening
5 kWh
- Covers a typical evening
- about 35%
- Federal rebate now
- $1,292
Good for: The lowest upfront cost.
Watch for: The grid still covers late evening.
What size battery do I need?
A battery is sized to your evenings, not your whole day. Three things decide it: how much power your home uses after the sun goes down, how much spare solar you have to fill it, and what you want the battery for. Try it with your own household below.
Step 1
Your day in power
Tell us about your household. Everything below updates to match.
kWh a day
Between 4pm and 8am your home uses about12.9 kWhThat's 69% of the 18.7 kWh it uses in a day (typical for 3–4 people in NSW).
Solar panels make power in the middle of the day, but most homes use most of theirs from late afternoon to breakfast: cooking, lights, TV, heating or cooling, hot showers. Without a battery, that evening power comes from the grid, while spare daytime solar is sold for a few cents. That evening figure is where the battery size starts.
Step 2
Try a battery size
Drag to see how much of your evening a battery covers, and what the federal rebate takes off.
94%
Covers a typical eveningabout 94%
Federal rebate, installed now$3,458
Rebate per kWh$256
Left for the grid each evening0.7 kWh
The federal rebate pays its full rate on the first 14 kWh of usable capacity, 60% of it from 14 to 28 kWh, and 15% from 28 to 50 kWh. So each extra kWh past 14 costs you more after the rebate: a 13.5 kWh battery earns about $3,458, and doubling it to 27 kWh adds only about $2,166. Coverage allows for the roughly 10% lost charging and discharging.
A battery also needs spare solar to fill it. If your panels only just cover your daytime use, a big battery spends winter half empty.
Three sizes, three goals
The planner compares three sizes against your evening use. For your household:
What changes the answer
- An electric car. Charging at home adds about 8 kWh a day. Charged overnight, your evening use goes from about 12.9 to 20.9 kWh, unless the car charges in the middle of the day from solar.
- Moving off gas. Electric hot water, heating and cooking add about 5 kWh a day. A heat-pump hot water system on a midday timer soaks up spare solar before the battery needs it.
- Where you live. The same household (3–4 people, home some days) uses about 10.3 kWh in Melbourne, 13.1 kWh in Brisbane, 12.5 kWh in Adelaide, 12.5 kWh in Perth, 19.1 kWh in Hobart, 16.3 kWh in Canberra and 14.5 kWh in Darwin in the evening (AER benchmarks: Tasmanian homes use the most power of any state).
- Solar you already have. An older 3 or 5 kW system may not fill a large battery in winter. Size the battery to the spare solar you actually have, or add panels.
- Backup in a blackout. That's about power (kW), not just storage (kWh). See inverters, below.
Inverters: the part people forget
A battery stores power as direct current (DC); your home runs on alternating current (AC). The inverter converts between them, and it decides two things the battery size doesn't: how much the battery can supply at once, and how it connects to your solar.
- Capacity and power are different. Capacity (kWh) is how much the battery holds. Power (kW) is how fast it can supply it, which the inverter sets. A 13.5 kWh battery that supplies 5 kW can run the house for hours, but not the oven, kettle and ducted air-conditioning all at once. When you ask for more than it can supply, the grid tops up the difference.
- Hybrid inverter or AC-coupled battery. A hybrid inverter runs your panels and battery together, which is common when you install both at once. An AC-coupled battery has its own inverter and sits beside your existing solar inverter, which often suits homes that already have solar. The Powerwall 3 AC is one example.
- Your existing solar. Older solar inverters often can't run a battery. You'll need either a new hybrid inverter (replacing the old one) or an AC-coupled battery. Ask which one your installer is quoting, and whether the cost is included.
- Your network sets limits. Networks cap how much an inverter can export, and single-phase homes face lower limits than three-phase ones. That's why the same battery can be listed at several inverter ratings: the Powerwall 3 is on the approved list at 5, 10 and 11.04 kVA.
- Blackout backup. Some setups back up the whole home, others only selected circuits, and some need an extra backup box. The inverter's power limit applies in a blackout too.
What can it run at once?
Tick what might be on at the same time on a winter evening. Fridge, lights and TV (0.5 kW) are always counted.
5 kW battery
About 6.6 kW. More than a 5 kW battery supplies at once: the grid tops up about 1.6 kW, and in a blackout it may trip or shed circuits.
Typical draws; yours may differ. 5 kW is a common rating, and some batteries supply more. Check the “continuous power” on the spec sheet.
Questions to ask your installer
- Will my existing inverter work with this battery, or does it need replacing, and is that in the price?
- What's the battery's continuous power in kW, and what will it run in a blackout?
- Is there an export limit on my connection, and am I single-phase or three-phase?
Quick guide by household: New South Wales, home some days
| Household | Daily use | 4pm to 8am | Sizes the planner compares |
|---|---|---|---|
| 1 person | 8.5 kWh | 5.9 kWh | 5 / 8 / 10 kWh |
| 2 people | 14.3 kWh | 9.8 kWh | 5 / 10 / 16 kWh |
| 3–4 people | 18.7 kWh | 12.9 kWh | 5 / 13.5 / 20 kWh |
| 5 or more | 24.7 kWh | 17 kWh | 8 / 16 / 25 kWh |
Typical use is the AER's benchmark for the household size in New South Wales. Your bill gives a better figure.
Get a size for your home
The planner takes your postcode, solar and bill, models your home hour by hour, and recommends a solar and battery size with costs after rebates. Your household answers from above are already filled in.
Questions
Is a 10 kWh battery enough for a family?
It covers most of an evening, not all of it. A family of four in Sydney uses about 12.9 kWh between 4pm and 8am on typical use; a 10 kWh battery delivers about 9 kWh after losses, so about 70% of a typical evening, with the grid covering the rest. To run the whole evening on solar, look at 13.5 kWh or more.
Is a bigger battery better?
Only if you'll fill it and use it. The federal rebate per kWh falls past 14 kWh, and a battery larger than your spare solar sits partly empty in winter.
Can I add more battery later?
Many batteries are modular, such as Powerwall 3 with expansion packs and stackable ranges like SigenStor, so it's often possible. But the federal rebate allows one battery claim per home, so capacity you add later may not earn any rebate. If you expect to need more, it's usually cheaper to size up at the start. Check with your installer.
Do I need a new inverter for a battery?
Not always. If you're installing solar and a battery together, a hybrid inverter runs both. If you already have solar, either your old inverter is replaced with a hybrid one, or you add a battery with its own inverter (AC-coupled) beside it. Ask your installer which they're quoting.
Does battery size change the rebate?
Yes. The federal rebate is worked out from usable capacity: full rate on the first 14 kWh, 60% from 14 to 28 kWh, and 15% from 28 to 50 kWh. A 13.5 kWh battery earns about $3,458 if installed now, at $38 per certificate.
Figures use the planner's model: AER typical use for the household size, a typical hour-by-hour pattern, and the federal rebate at $38 per STC. See how long a battery lasts overnight and the rebate calculator for a specific battery.