How big a home battery do you need?

Start with the right question

The mistake most people make is sizing a battery to their whole daily usage. You do not need to. While the sun is up, your panels are already running the house. A battery's job is the part of the day your solar cannot cover: from late afternoon, through the evening, and overnight until the panels wake up again.

So the question is not "how much power do we use?" It is "how much do we use when the sun is down?"

Step 1: find your evening and overnight usage

The best source is your own data. Two easy ways to get it:

• Your retailer's app or online account. Most show your usage in half-hour blocks if you have a smart meter. Add up a typical day from about 4pm to 8am.
• Your bill. If it splits usage into peak, shoulder and off-peak, the evening peak and overnight off-peak together are a rough guide.

Use an ordinary day, not your worst one. Then think about what is changing. An EV charged at night, a new pool pump or moving to reverse cycle heating will all move the number up.

Step 2: make sure your solar can fill it

A battery only helps if it gets full. Your panels have to run the house during the day and have enough left over to charge the battery.

Across most Australian cities, each kW of panels makes somewhere around 3.5 to 4.5 kWh a day averaged over a year, less in winter and more in summer. So a 6.6 kW system makes very roughly 25 kWh on an average day. If the house uses 10 of that during daylight, there are about 15 kWh left to charge a battery. That comfortably fills a 10 to 14 kWh battery most of the year, but not a 30 kWh one.

If your roof is small or shaded, or your solar is an older 3 kW system, a bigger battery may sit half empty. Sometimes the better spend is more panels first.

Step 3: read the right numbers on the spec sheet

• Usable capacity (kWh) is what you can actually use. The nominal or total capacity is a bigger number that includes a reserve you cannot touch. Compare batteries on usable kWh, and the rebate is worked out on usable too.
• Power rating (kW) is how fast it can deliver. A 10 kWh battery rated at 5 kW can run 5 kW of appliances at once. If the oven, air conditioning and kettle are all on at 6pm, a low power rating means you still buy some from the grid, even with charge left.
• Backup. Not every battery keeps the lights on in a blackout. That depends on the inverter and how the backup circuits are wired. If blackout protection matters to you, say so at the start, because it changes the install.

Step 4: weigh the rebate tiers

Since 1 May 2026 the federal rebate pays the full rate on the first 14 kWh of usable capacity, 60% on the next 14 kWh and 15% after that, up to 50 kWh. The full breakdown is in The 2026 home battery rebate, explained.

For sizing, the takeaway is simple. Up to about 14 kWh usable, the rebate is pulling hard. Past 28 kWh, extra capacity is almost entirely at your own cost, so it has to pay for itself.

Step 5: decide how you want to use it

The same home can need a different size depending on what the battery is for.

That last one is where Sunnn spends most of its time. On a wholesale plan a battery earns by selling when prices spike, which only works if it still has charge left after running your evening. That usually means sizing a little above your evening usage, and a power rating high enough to sell hard for a short window.

A worked example

This is illustrative, not a quote. A family of four in Melbourne uses about 18 kWh a day. Their smart meter data shows about 11 kWh of that is between 4pm and 8am. They have 6.6 kW of solar.

• Evening need: about 11 kWh, so 11 to 14 kWh usable covers it with headroom.
• Solar check: in Melbourne 6.6 kW makes very roughly 24 kWh on an average day. With about 7 kWh used in daylight, that leaves plenty to fill the battery most of the year, though short winter days will fall short.
• Rebate: 14 kWh usable sits right at the top of the full-rate tier.

A battery around 14 kWh usable is a strong fit. If they were planning an EV, or wanted to trade on a wholesale plan, the case for going to around 20 kWh gets stronger, with the extra 6 kWh earning 60% of the rebate rate.

Get it sized properly

The quickest way to see your own numbers is the savings calculator. For a proper design, check your eligibility and we will size solar and battery from your actual usage, show the rebate as a line on the quote, and set the battery up to suit the plan you choose. Savings figures are modelled estimates, not guarantees.