Buying solar

What size solar battery do you actually need?

Enough to cover what your house uses between sunset and sunrise, and not much more. For most Australian households that is 10 to 13 kilowatt-hours of usable storage. Large homes, all-electric homes and EV owners push towards 20. Bigger than your overnight usage sits idle, and the federal rebate pays proportionally less above 14 kWh.

By Kieron Taylor, Founder, Solar Select · Updated · 7 min read

The 30-second take

  • Size to your overnight usage. A working household uses 60–70% of its electricity outside solar hours; on 20 kWh a day that is 12–14 kWh to cover.
  • 10 kWh suits a 2–3 person home using 12–18 kWh a day. 13 kWh suits a 4-person home on 18–22. 20 kWh suits a large or all-electric home, an EV, or a house that wants a full night of backup.
  • 2026 installed prices: a 10 kWh battery about $8,000–$10,000; a 20 kWh battery roughly $10,000–$13,000 depending on brand and whether an inverter is included. All figures after the federal rebate.
  • The federal rebate pays the full rate on the first 14 kWh, 60% from 14 to 28 kWh and 15% above that, so a 20 kWh battery earns less rebate per kWh than a 13.
  • A battery never removes the daily supply charge, and it only saves money on kilowatt-hours you would otherwise buy. Oversizing does not save more; it costs more.

The sizing rule

A home battery does one job: it moves the solar you make at midday into the hours you actually use electricity. So the right size is set by how much you use when the sun is down, not by how much you use in total and not by how big your solar system is.

Start from the average daily usage on your bill. A working household uses only 30 to 40 per cent of its electricity during solar hours; the remaining 60 to 70 per cent is the evening, night and early morning load a battery can cover. A home on 20 kWh a day has roughly 12 to 14 kWh of overnight load, and a battery sized to that — 13 kWh usable — covers it on an ordinary day.

Then check the solar can fill it. A 6.6 kW system makes about 25 kWh a day averaged across the year, of which the house uses perhaps 7 to 9 directly, leaving 16 to 18 to charge a battery. That comfortably fills 13 kWh in summer and mostly fills it in winter. A 20 kWh battery behind a 6.6 kW system will rarely be full between May and August.

“The battery is sized to the night, and the solar is sized to fill it. Get those in the wrong order and you buy storage that sits half empty.”

Which size fits which household

The regulators’ benchmark household uses 10.7 to 13.4 kWh a day, but real homes spread from under 10 to over 25. Match yours to the closest row.

  • Around 10 kWh usable — 1–3 people using 12–18 kWh a day, gas hot water or a heat pump, no EV. Covers a normal evening with a small margin. The size most Australian households buy.
  • Around 13 kWh usable — a 4-person home on 18–22 kWh a day, electric hot water on a daytime timer, ducted air conditioning used in the evening. The sweet spot for a family home with a 6.6 kW or larger array.
  • Around 20 kWh usable — a large or all-electric home on 25 kWh or more, an EV charged overnight, a pool, or a household that wants a full night of whole-home backup in a blackout. Needs 10 kW of solar or more to fill in winter.
  • Beyond 20 kWh — rarely justified on a single-phase home. The rebate drops to 60% and then 15%, the solar cannot fill it, and the money is usually better spent on more panels or a heat pump.

Backup changes the answer. If keeping the fridge, lights and internet running through an outage matters more than the payback, size for the longest outage you want to ride through rather than for an average night, and confirm the battery and inverter are wired for backup at all — many are not by default.

What each size costs in 2026

For the size most households buy — around 10 kWh usable — expect roughly $8,000 to $10,000 fully installed in 2026, federal rebate deducted. Across the whole market, from small units to large high-end systems, installed prices run from about $4,000 to $13,000 or more.

At 20 kWh the hardware alone spans a wide range. SolarQuotes’ August 2026 comparison table lists 19–20 kWh batteries from about $6,500 to $10,000 after rebate before installation, and puts installation at $3,000 or more on top. That lands a 20 kWh system around $10,000 to $13,000 installed, higher if the battery needs a new hybrid inverter or whole-home backup wiring. Per kilowatt-hour, the larger battery is cheaper; per dollar saved, it is usually not.

  • Compare dollars per usable kWh, not headline price. A battery advertised at 16 kWh may have 14 usable, and the rebate follows the usable figure.
  • Check whether an inverter is included. AC-coupled batteries work with your existing solar inverter; DC-coupled ones need a hybrid inverter, which can add several thousand dollars if yours has to be replaced.
  • Backup scope is a separate line. No backup, essential circuits or whole-home are three different jobs with three different prices.
  • One rebate per address, permanently. If a previous owner claimed it, it is gone.

How the rebate taper changes the maths above 14 kWh

The federal Cheaper Home Batteries Program pays roughly a quarter off the installed cost, delivered as certificates your installer applies upfront. From 1 May 2026 the factor is 6.8 certificates per usable kilowatt-hour, dropping to 5.7 on 1 January 2027.

The factor applies in full only to the first 14 kWh. From 14 to 28 kWh it pays 60 per cent, and from 28 to 50 kWh 15 per cent. So a 13 kWh battery earns the full rebate on every kilowatt-hour, while a 20 kWh battery earns full rate on 14 and 60 per cent on the remaining 6. In dollars, the 20 kWh battery gets a larger rebate in total but a smaller one per kilowatt-hour — which is one more reason the step from 13 to 20 needs a real overnight load to justify it.

Two mistakes that cost the most

The first is sizing the battery to the solar system. "You have 10 kW of panels so you need 20 kWh of storage" is a sales line, not a sizing method. The battery is sized to what the house uses at night; the solar only has to be big enough to fill it.

The second is treating the whole bill as the saving. A battery never removes the daily supply charge, which is $1.30 to $2.70 a day depending on your network, and it only saves on the kilowatt-hours you would otherwise have bought after dark. A household with a $400 quarterly bill and 13 kWh of overnight load saves on that load, not on the $400.

The Clean Energy Regulator inspected 1,278 home battery installations between July 2025 and April 2026 and classified more than 60 per cent as substandard. Size is a spreadsheet question; who wires it in is the one that decides whether the battery is still working in year eight. Get matched with 2-3 vetted solar installers — typically 2+ years in business, with a track record of decent reviews — give them your last four bills, and ask each of them to size the battery to your overnight usage rather than to their stock.

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Sources

Every quantitative claim on this page traces to one of the sources below — government, manufacturer, or independent AU industry reference. Last verified 2026-09-18.

We use AI to help research and draft. Claims are traced to the sources below and dated. Where we haven’t got a source, we say so on the page — treat that as a starting point and do your own research.

General information only. It doesn’t take your circumstances into account and isn’t legal, financial, insurance or electrical advice. Rebates, tariffs, network rules and product terms change — confirm anything you plan to rely on with your installer, insurer, retailer or distributor, and see our terms.

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