What size battery do I need? A simple way to work it out

There’s no single right size. A battery earns its keep by storing solar you’d otherwise export and covering the power you’d otherwise buy at night, so the right size depends on how much of each you have. Solar Victoria says the choice mostly depends on the size of your solar system and your overnight use, plus whether you want backup in a power outage.

The steps below use numbers from your own bills. You’ll need about ten minutes and a recent bill, or access to your retailer’s app.

Capacity and power are two different numbers

Battery quotes list two figures, and both matter. Capacity, in kilowatt-hours (kWh), is how much energy the battery holds. Power, in kilowatts (kW), is how fast it can deliver it, which limits how many appliances it can run at once.

Capacity also comes in two versions. Nominal capacity is what the cells hold; usable capacity is what you can draw on. For lithium-ion batteries, energy.gov.au puts the gap between usable and actual capacity at 5 to 10%, and a similar share is lost each time energy goes in and out. Always compare usable capacity.

Step 1: find out what you use after sunset

This is the energy a battery can replace. If you have a smart meter, your retailer’s app or online account shows your use by the hour. Add up a typical day from about sunset to sunrise. Our guide to reading your electricity bill shows where to find the numbers.

Step 2: find out how much solar you export

This is the energy a battery can store. Your bill shows the kWh you exported; divide it by the days in the billing period. If you export little, a battery has little to store: Your Home notes that a well-designed solar and battery system matters more than a large battery.

Step 3: take the smaller number

A battery can only shift the smaller of the two: there’s no point storing more than you use at night, or buying more capacity than your solar can fill. The same Solar Victoria guide suggests a battery you’ll regularly charge and discharge to the recommended level, without cycling it fully several times a day.

Three illustrative homes

These figures are made up to show the method; use your own. Round up a little if you expect your night-time use to grow, for example with an electric car or a heat pump.

Step 4: check the power covers what you run at once

Capacity tells you how long a battery lasts; power tells you what it can run. If you switch on more than the battery can supply, the rest comes from the grid. A 5 kW battery won’t cover an oven, a kettle and an air conditioner running together.

• Check the kW rating: single home batteries commonly deliver 5 to 10 kW continuously. The figure is on the data sheet.
• Big evening loads: electric cooking, ducted air conditioning or a pool pump running together may need more power, or two units.
• Three-phase homes: check whether the battery covers one phase or all three.

Step 5: decide whether you want backup

Backup depends on how the system is set up. Even some battery systems won’t power a home during an outage, and single-phase batteries and hybrid inverters only back up one phase.

If backup matters, list what you’d need to run (fridge, lights, internet, a pump), and how long for. Backing up a few essential circuits for a night needs far less than running the whole house for days.

How the federal discount changes the sums

Since 1 May 2026, the Cheaper Home Batteries Program discount pays the 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 kWh above 14 costs you more after the discount. Our battery rebate calculator shows the discount for any size and install date.

Retailers have to explain the battery’s size relative to your solar, along with the expected payback and savings. Ask for those figures in writing.

Get a second opinion

Your Home recommends checking a retailer’s advice with an independent tool. The free SunSPOT solar and battery calculator estimates the cost, savings and payback for your address. Then work through whether a battery is worth it for you.

When you’re ready to compare models, see our best home batteries for different needs, and find battery retailers that cover your postcode.

This guide is general information, not financial or legal advice. Rules, rebates and prices change, so check the linked sources before you decide. Facts checked 22 October 2026.