Step 1: find your evening use
Look at your interval data, which most retailers show in their app or let you download, and add up the power you import from about sunset to sunrise on a typical day. Do this for both summer and winter. That figure is your starting size.
Step 2: decide what else the battery is for
- Blackout backup needs a battery and inverter that support backup, plus a reserve you keep unused. It's a separate decision from saving money.
- EV charging from a home battery can add a lot of daily kWh. Charging the car directly from solar in the daytime is usually more efficient.
Step 3: check the rebate taper
The discount applies at the full STC factor up to 14 kWh, 60% from 14 to 28 kWh and 15% from 28 to 50 kWh. So a step from 13 kWh to 20 kWh adds capacity at a smaller discount. Run both sizes through our rebate calculator and payback calculator.
Step 4: match the inverter
Capacity (kWh) is how much energy the battery stores. Inverter power (kW) is how fast it can deliver it. A large battery behind a small inverter can't power your evening peak, and the grid fills the gap.
How big do most households need?
ABC News reported a Smart Energy Council analysis finding that over 90% of households could cover their evening peak with a battery as small as 6–7 kWh, smaller than the popular ~13 kWh models. Households with electric heating, pools, EVs or big evening loads need more, which is why your own data matters.