What Size Solar Battery Do I Need?
Size a battery to your evening and overnight use, and make sure your solar can fill it. A step-by-step guide using your bills, meter data and the rebate bands.
The right battery size is the amount of electricity your home uses after the sun goes down and overnight, as long as your solar panels can produce enough spare energy during the day to fill it. There is no single right size. Your own bills and smart meter data are a far better guide than any average.
Step 1: find out how much electricity you use
Your bill shows your average daily use in kWh. The federal government's energy.gov.au suggests checking bills from different seasons, because use changes through the year. Queensland homes use the most in summer, when air conditioning runs, so a battery sized on a winter bill can fall short in summer.
For a rough benchmark, the Australian Energy Regulator's figures for Brisbane and the Gold Coast show average grid use of about:
- 1 person: 9 kWh a day
- 2 people: 14 kWh a day
- 3 people: 17 kWh a day
- 4 people: 21 kWh a day
- 5 or more people: 24 kWh a day
These are averages of electricity bought from the grid and date from 2020. Homes with pools, electric hot water, ducted air conditioning or an electric vehicle can use much more.
Step 2: work out when you use it
A battery covers the hours when your solar is not producing, mainly late afternoon, evening and overnight. That is the number to size for, not your total daily use.
If you have a smart meter, you can ask your electricity distributor (Energex in most of South East Queensland) for your interval data. It shows your use every 15 or 30 minutes for the past year, which is the most accurate way to see how much you use between sunset and sunrise.
Step 3: check your solar can fill the battery
A battery only helps if there is spare solar to store. Energy.gov.au estimates a 6.6 kW solar system in Brisbane produces about 28 kWh on a sunny day. Your home uses some of that during the day, and the battery gets what is left. On cloudy days, it gets less.
If your solar system is small, a large battery may rarely reach full charge. In that case, adding panels may help more than adding storage. The government's own advice is that bigger is not always better: a battery too large for your solar or inverter limits the benefit.
Usable capacity vs total capacity
Batteries are sold with two figures:
- Total (nominal) capacity: everything the battery can hold.
- Usable capacity: what you can actually draw on. For lithium batteries it is usually 5% to 10% less than the total.
Always compare usable capacity. For example, a Tesla Powerwall 3 has 13.5 kWh usable from 14 kWh total. Sigenergy's modules are sold as 5, 6, 8 and 10 kWh, but their usable capacities are 5.2, 5.84, 7.8 and 8.76 kWh.
You also lose about 10% of stored energy as heat in charging and discharging, so a 10 kWh battery delivers a little under 10 kWh to the house.
Capacity vs power
Capacity (kWh) is how much energy the battery holds. Power (kW) is how much it can deliver at once. A battery with plenty of capacity but low power cannot run the air conditioner, oven and kettle together, and your home will top up from the grid. If you have large appliances that run in the evening, check the battery's continuous power rating as well as its capacity.
Sizing for backup
If you want power during blackouts, think about which circuits you need to keep running and for how long. A battery can back up the whole house, selected essential circuits, or one phase of a three-phase home, depending on how it is set up. Some systems also let your solar keep recharging the battery during an outage. Our guide to storm season and battery backup covers this in more detail.
How the federal battery rebate affects size
The Cheaper Home Batteries Program discount is based on usable capacity, and since 1 May 2026 it tapers as batteries get bigger:
- up to 14 kWh: full rate
- 14 to 28 kWh: 60% of the rate for each extra kWh
- 28 to 50 kWh: 15% of the rate for each extra kWh
- above 50 kWh: no discount on the extra capacity
The rate also steps down every six months. It falls from 6.8 to 5.7 certificates per kWh on 1 January 2027. Our battery rebate page explains how the discount is applied.
Only one battery per property can claim the rebate. If you claim it now and add modules later, the extra capacity will not attract the discount. That is worth considering if you are tempted to start small and expand.
Putting it together
- Find your average daily use from bills across seasons.
- Estimate your evening and overnight share, ideally from smart meter data.
- Choose a usable capacity close to that figure.
- Check your solar produces enough spare energy to fill it.
- Check the battery's power rating suits your evening appliances.
- Decide whether you need backup, and for which circuits.
- Weigh the rebate bands and whether you might expand later.
For more on choosing between batteries, see choosing the right solar battery storage for your home, and for what to expect over time, how long solar batteries last.
Frequently asked questions
Too small:
- it runs flat early in the evening most days
- you still buy a lot of power during the 4pm to 9pm peak
Too large:
- it rarely reaches full charge, even on sunny days
- a large share of its capacity goes unused overnight
Most current batteries are modular. A Powerwall 3 can be expanded to 54 kWh, and SigenStor takes up to six modules per controller. Expansion limits are specific to each brand, and later add-ons do not get the federal discount if the original battery already claimed it.