Start with your usage, not a battery size
Look at your electricity bill for your average daily usage in kilowatt-hours, then think about how much of that happens after the sun goes down. A battery's job is mostly to shift your daytime solar into your evening and overnight use, so the number that matters is your off-solar consumption, not your total.
A rough way in: if you use 20kWh a day and roughly half of that is in the evening and overnight, a battery in the 10 to 12kWh usable range covers most of it. Bigger only helps if you actually use more after dark or want deeper backup.
Factor in your solar
A battery is only as useful as the surplus solar you have to fill it. If your panels comfortably cover your daytime use and still export a healthy amount, you have surplus to store, and a battery captures it instead of selling it back cheaply. If your solar is small or your daytime use is high, there may be little left to charge a large battery, which caps the sensible size.
This is why sizing is a two-sided sum: your evening demand on one side, your spare solar on the other. The battery that fits sits where those two meet.
Bill savings or blackout backup?
Your goal changes the sizing as much as your usage does.
- Mainly bill savings: size to your evening and overnight use, and stop there. Extra capacity you rarely fill rarely pays back.
- Mainly backup: you may size a little larger and prioritise batteries and setups that actually run your home during an outage, which not all do.
- A bit of both: the common case, and where sizing from real usage data earns its keep.
The rebate taper: why 14kWh is a marker
The federal Cheaper Home Batteries rebate is strongest on the first portion of capacity and tapers as systems get larger, with the step often described around 14kWh of usable capacity. Indicatively it is worth roughly $252 per usable kWh at the current spot price, applied at the point of sale, though the STC price floats so treat any figure as a guide.
The practical upshot is that the first 10 to 14kWh tends to carry the best rebate value, and going much larger is a decision about your actual needs rather than chasing subsidy. An installer can model the sweet spot for your home.