Choosing a battery size is the most important part of buying a battery. Too small, and you still buy lots of power at night. Too big, and you pay for storage that sits empty most of the year. Here is a simple way to work out the right size for your home.
Key points
- Size your battery to the power you use after the sun goes down, not to the size of your house.
- Your solar system must make enough spare power to fill the battery.
- Many Victorian homes suit a battery of about 10 to 14 kWh.
- The federal discount is highest on the first 14 kWh, so bigger batteries get less support per kWh.
Step 1: Find out how much power you use at night
A battery only saves you money when it powers your home instead of the grid, which is mostly from late afternoon to the next morning. So the key number is your evening and overnight use.
Victorian homes have smart meters, so you can usually download half-hourly usage data from your retailer or your electricity distributor’s online portal. Add up what you use between about 5pm and 7am. Many homes use somewhere between 6 and 15 kWh in that time.
If you cannot get the data, look at your bill. Many Victorian homes use around 10 to 15 kWh a day in total, and often more than half of that is outside daylight hours.
Step 2: Check your solar can fill the battery
A battery needs spare solar to charge. If your solar system is small or your home uses most of it during the day, a big battery will rarely fill up.
In Melbourne, each 1 kW of panels makes about 3.6 kWh a day on average, but much less in winter. A 6.6 kW system might make 30 kWh on a good summer day and under 12 kWh on a dull winter day. If you want a big battery, you may also need more panels.
Step 3: Match the size to your home
| Home | Typical evening use | Battery size to consider |
|---|---|---|
| Couple, out during the day | 5 to 8 kWh | 5 to 10 kWh |
| Family home | 8 to 14 kWh | 10 to 14 kWh |
| Large or all-electric home, pool or EV | 15 kWh or more | 14 to 25 kWh, often with more solar |
Step 4: Think about the federal discount tiers
The federal Cheaper Home Batteries Program gives full support for the first 14 kWh of usable capacity. From 14 to 28 kWh it pays only 60% of the rate, and above 28 kWh only 15%. For most homes, a battery of up to about 14 kWh gives the best value. Read our battery rebate guide for the numbers.
Step 5: Decide if you want blackout backup
If you want your home to keep running in a blackout, you need a battery that supports backup and the right extra equipment. Backup does not change the size much, but it does change which battery you choose. Whole-home backup with a Tesla Powerwall 3 needs a backup gateway. Enphase systems need a system controller.
Common sizing mistakes
- Buying the biggest battery you can. It may never fill up in winter, and extra kWh above 14 get less discount.
- Ignoring winter. Your solar makes far less in June and July.
- Forgetting future changes. An EV, heat pump or induction cooktop will change your evening use. Modular batteries such as Enphase can grow with you.
- Confusing nominal and usable capacity. Always compare usable kWh.
Let us size it for you
We look at your smart meter data, your solar output and your plans before we recommend a battery. We install Tesla Powerwall 3, Enphase, Sigenergy and Sungrow batteries across the Mornington Peninsula, Frankston and Bayside. See our solar battery installation page or call 1300 374 786.
Frequently asked questions
Is a 10 kWh battery enough?
For many family homes, yes. It covers a typical evening and part of the night.
Can I add more battery later?
Some batteries are modular. Enphase batteries can be added in 5 kWh steps, and Tesla Powerwall 3 can take expansion units.
Will a battery run my home all night?
It can if it is big enough for your overnight use and your solar fills it during the day. In winter, you will usually still buy some power from the grid.
