What size home battery do I need?
Most people are sold a battery on its headline size. The right size depends on one thing: how much power your home uses after the sun goes down. Here is how to work it out in a few minutes.
The short answer
- Under 20kWh a day: a 10kWh battery is usually enough.
- 20 to 35kWh a day: about 15kWh. Over 35kWh: about 27kWh or more.
- Installed, before rebates: about $7,650 to $13,500 for 10kWh, with $2,520 off from the federal discount for installs to December 2026.
Start with your daily usage
Your bill shows your average daily use in kWh. Our calculator suggests a battery size from it[1]:
- Under 20kWh a day: 10kWh battery.
- 20 to 35kWh a day: about 15kWh.
- 35 to 40kWh a day: about 27kWh.
- Over 40kWh a day: about 40kWh.
That is a starting point. What really matters is how much of that use falls in the evening and overnight, because that is what a battery covers.
How long does a 10kWh battery last?
You can't use every kWh a battery holds. Our calculator assumes 90% is usable, so a 10kWh battery gives about 9kWh[1].
- At 1kW, a typical evening with lights, TV, fridge and cooking, that is about 9 hours.
- At 2kW, with a dryer, oven or air conditioner running, about 4 to 5 hours.
- Ducted heating or charging an electric car can empty it in an hour or two.
If your battery runs flat by midnight most nights, it is too small for you. If it is still half full at sunrise, it is too big.
Too big costs more than too small
Every extra kWh you never use is money spent for nothing. A battery only saves money when it is emptied and refilled, ideally every day. Start with your evening use, and add capacity only for something you will really use: an electric car, a planned heat pump, or backup during outages.
Your solar has to fill it too. A small solar system on a winter day may not charge a large battery.
What each size costs
Installed prices our calculator uses, before and after the federal battery discount for installs to December 2026[1]:
- 10kWh: $7,650 to $13,500 before, $2,520 off.
- About 15kWh: $12,100 to $18,000 before, $3,680 off.
- About 27kWh: $16,470 to $28,000 before, $5,490 off.
- About 40kWh: $21,200 to $33,450 before, $6,100 off.
The discount per kWh falls every six months, next in January 2027, so the same battery will cost more after rebates next year[2].
Why two quotes for the same job can be thousands apart
Installers price the same system very differently. These are the installed price ranges our calculator works from, before rebates: the low end is what a competitive installer charges, and the high end is what a poor quote looks like.
| System | Competitive quote | Poor quote |
|---|---|---|
| 6.6kW solar system | $8,000 | $10,000 |
| 10kWh battery | $7,650 | $13,500 |
| About 15kWh battery | $12,100 | $18,000 |
- The gap usually comes from margin, the brand of panel, inverter or battery, and extras such as a switchboard upgrade or backup circuit, not from a better install.
- Get at least two or three quotes, and ask for each to be itemised: hardware brands and models, size, rebates taken off, and anything extra.
- Compare batteries on price per usable kWh, not the headline price.
Get the numbers for your own home
Two minutes of questions about your bill, roof and routine gives you the system size, what it should cost after rebates and what it saves. Your report shows on screen straight away, and no installer sees anything unless you ask.
Run my numbers, freeCommon questions
What size battery do I need for a 3 or 4 bedroom house?
It depends on use, not bedrooms. A home using under 20kWh a day usually suits a 10kWh battery; 20 to 35kWh a day, about 15kWh.
How long does a 10kWh battery last?
About 9kWh is usable. That is about 9 hours at a 1kW evening load, or 4 to 5 hours at 2kW.
Is a bigger battery better?
Only if you use it. A battery that is still half full at sunrise cost you money it will never pay back.
How much is a 10kWh battery in 2026?
Our calculator uses $7,650 to $13,500 installed, before the federal discount of $2,520 for installs to December 2026.
Sources
- Aussie Solar Info, How the calculator works: installed prices, battery sizes and the federal battery discount used by this site's calculator. Read 4 November 2026.
- Clean Energy Regulator, Solar batteries (Cheaper Home Batteries Program): the discount factor falls every six months, to 5.7 for January to June 2027. Read 26 October 2026.