What size solar battery do you need?

A battery earns its keep by storing solar you would otherwise export and supplying the house when the panels stop. So two figures bound its size, and the smaller one wins.

1. Evening and overnight use. Your daily use times the share that falls after sunset and before the panels restart in the morning. A battery bigger than this sits partly unused. If you don’t know your daily use, the calculator can take the Australian Energy Regulator’s average for your household size and area instead.
2. Average solar output. Panel size times the Clean Energy Regulator’s rating for your postcode’s zone, converted to a daily figure.
3. Solar left over. Output minus what the house uses while the panels produce. That surplus is all a solar-charged battery can take in.
4. Losses. Charging and discharging loses some energy, so the leftover is multiplied by a published efficiency figure before it is compared.
5. The smaller of the two is the usable capacity your numbers support. The calculator says which one set it.
6. Backup reserve, if any. Charge you keep back for blackouts is added on top, because everyday use doesn’t draw on it.

It works in usable kilowatt-hours only. It does not price a battery, estimate a bill or project a saving: those depend on your electricity plan, which this page does not model.

Every physical figure comes from a published document; the three settings we chose ourselves are marked as ours. The solar zone ratings are the output the Clean Energy Regulator deems each kilowatt of panels to produce in a year, used to calculate small-scale technology certificates. The table is dated 1 Jan 2020.

The 89% is the published solar to battery to home/grid efficiency for the Tesla Powerwall 3, stated for typical solar-shifting use, 25°C, beginning of life, 3.3 kW charge/discharge. One manufacturer's published figure, used as a stand-in for battery losses. Other batteries, temperatures and older batteries differ.

The household-size estimate is the Australian Energy Regulator’s consumption benchmark for your postcode’s climate zone and state (in South Australia, its own zones), published for comparing household bills. The AER gives a figure for each season; the calculator uses the yearly average of the four, so a bill from your highest-use season will usually read higher. That is winter in most areas, and summer in the warm north (Queensland and northern NSW, climate zones 1 to 3). Where it had too few homes to tell sizes apart, the AER grouped some household sizes, so three, four and five-plus people can share one figure. It covers NSW, the ACT, Queensland, Victoria, South Australia and Tasmania. Gross electricity imported from the grid, averaged over homes with and without solar. Solar a home uses directly is not counted. A home that already has solar therefore uses more in total than the benchmark shows.

How much of your electricity you use in the evening and overnight depends on your household, so it is yours to set. The three answers to “is anyone usually home during the day?” map to rough starting points we picked, with the percentage shown; they are not measured households.

• Season. The zone rating is a yearly average. Winter output is lower, so a battery sized on the average fills less often in June than in December.
• Your roof. Orientation, shading and an inverter smaller than the panels all reduce output. An installer’s yield estimate for your roof beats the zone rating.
• Backup reserve. If the battery keeps charge back for blackouts, that capacity is not available for daily use. Enter it in the calculator and it is added on top of the result.
• Battery ageing. Every capacity here is the figure for a new battery. Usable capacity falls over the years, so a battery sized exactly to your evening use will cover a little less of it later. See how long solar batteries last.
• New loads. An EV, a heat pump or induction cooking changes both totals. Run it again with the use you expect, not last year’s.
• A VPP. A virtual power plant may also charge or discharge the battery for the grid. See the VPP comparison for how each program uses it.

For installations from 1 May 2026, the Cheaper Home Batteries Program calculates certificates on usable capacity, tapered by band: the full factor up to 14 kWh, 60% of it for each kWh above 14 up to 28 kWh, 15% of it for each kWh above 28 up to 50 kWh, according to the Clean Energy Regulator. The battery must be connected to solar and have 5–100 kWh of nominal capacity, as the eligibility rules set out.

The calculator shows which bands your result falls in. It does not convert that into dollars, because the certificate factor steps down by installation date and the certificate price moves. The federal battery rebate guide works through the current factor.

• Usable, not nominal. Compare the quote’s usable capacity with the result. Nominal capacity is larger and is not what the house can draw.
• Your real split. With a smart meter, your retailer can give you interval data showing when you use electricity. Replace the estimate with the share you actually use in the evening and overnight.
• Room to grow. A modular battery can add capacity later. Ask whether added modules need the same firmware, age or inverter.