It is the question every homeowner asks, and it is the one most likely to get a vague answer. Bigger is not automatically better. A battery that sits half charged every night is money parked on your wall doing nothing, and an undersized one leaves you buying grid power at peak rates. The right size is the one that matches how your household actually uses electricity after the sun goes down.
Here is how to work it out, what 10 kWh, 13 kWh, and 20 kWh each realistically cover, and why the changed 2026 rebate nudges most homes toward a particular size.
Start with your overnight usage, not your total usage
Your daily electricity use is not the number that matters. During the day, your solar panels are running the house directly. A battery only earns its keep from late afternoon through to the next morning, so the figure to find is how much power you use once solar stops producing.
You can pull this from your electricity bill or your meter data. Most Australian homes use roughly 40 to 60% of their daily electricity in the evening and overnight. As a rough guide:
- A smaller household using around 16 to 20 kWh a day might use 8 to 11 kWh of that after dark.
- A busy family home using 25 to 35 kWh a day might use 13 to 18 kWh overnight.
- A large home with a pool, ducted air con, and an EV can easily push past 20 kWh overnight.
The goal is a battery that comfortably covers a normal evening and overnight run, with a little headroom, without paying for capacity you will rarely cycle. It is also worth adding a small buffer of around 20% for capacity that fades slightly over the battery’s life. If you are unclear on the units, our explainer on kW vs kWh clears it up.
10 kWh: the right fit for smaller and efficient homes
A 10 kWh battery suits a couple, a small family, or an energy conscious household. It will typically carry your evening cooking, lighting, entertainment, and standby loads through to morning, and it pairs well with a solar array around 6.6 kW.
Where 10 kWh runs short is on heavy days. Run ducted air conditioning hard on a summer evening, or add an EV charging overnight, and you will drain it and start pulling from the grid before sunrise. If your usage is modest and stable though, a 10 kWh battery is often the most economical choice, because you are cycling most of it every single day. It also comfortably qualifies for the top tiers of most Virtual Power Plant programs, which can earn bill credits.
Good match if: you use roughly 8 to 11 kWh overnight, have no EV, and want a system that pays its way through daily full cycling.
13 kWh: the popular all rounder
Around 13 kWh is where a lot of Australian families land, and it is easy to see why. It covers a typical family evening, including some air conditioning, and still has reserve for a cooler than expected night or a load of washing after dinner. A 13.5 kWh battery is roughly enough to run an average home through the evening and keep essentials going during a blackout.
This size pairs naturally with a larger solar system in the 8 to 10 kW range, which can refill the battery even on a partly cloudy day. It is the comfortable middle: not so small that you ration, not so large that half of it sits idle.
Good match if: you use around 12 to 15 kWh overnight, run air con most summer evenings, and want backup that covers more than just the bare essentials.
20 kWh: for large homes, EVs, and heavy demand
A 20 kWh battery is a different category. It is built for large homes with high overnight demand: think multiple air conditioning zones, a pool, a big family, or an EV that charges at home. At this size you usually want a solar array of 10 kW or more so you can actually refill it day to day, otherwise the top slice of the battery rarely gets used.
Twenty kilowatt hours also buys longer blackout resilience, which appeals in areas prone to outages. The thing to be honest about is cycling. If your overnight use is really 14 kWh, a 20 kWh battery means 6 kWh sits unused most nights, and under the current rebate you are paying closer to full price for that upper slice. Size to your genuine demand, not to a round number.
Good match if: you use 16 kWh or more overnight, have an EV or a pool, run heavy air conditioning, and have the solar to keep it topped up.
A quick way to sanity check the pairing
Every 1 kW of solar generates roughly 4 to 5 kWh a day across a year in most of Australia. So a 6.6 kW system produces around 26 to 33 kWh on a good day. After your daytime household use, whatever is left is what fills the battery. If the surplus cannot fill the battery on an average day, the battery is too big for the array. A rough rule: pair 10 kWh with 6.6 kW or more, and step up to a 10 kW array once you go past 13 kWh of storage. Our comparison of a 6.6kW vs 10kW system is a good next read, along with the best panels to pair with a new battery.
Why 14 kWh is the 2026 sweet spot
The federal rebate reshaped this decision on 1 May 2026. The Cheaper Home Batteries Program switched from a flat discount to a tiered one, and it now steps down every six months rather than annually.
The important number is 14 kWh. The full rebate rate applies to the first 14 kWh of usable capacity. Above that, support drops steeply, with the 14 to 28 kWh band funded at a much lower rate and anything past 28 kWh lower again. As of mid 2026 the rebate is worth roughly $250 per usable kWh across that first 14 kWh, and the next step down is set for 1 January 2027. The full detail is in our federal battery rebate guide.
What this means in practice: a battery sized around 14 kWh captures the strongest rebate per kilowatt hour before the taper kicks in. That is why 13 to 14 kWh has become the value size for most households. A 10 kWh battery still earns the full rate on every kWh, so it remains efficient for smaller homes. A 20 kWh battery is not disqualified, but you should be genuinely using that capacity to justify paying less subsidised rates on the top end.
To claim the rebate at any size, your system needs to be VPP capable, use CEC approved products, and be installed by a Solar Accreditation Australia accredited installer.
Do not forget your solar array
A battery is only as useful as your panels’ ability to fill it. There is no point in a 20 kWh battery behind a 5 kW solar system that cannot recharge it on a cloudy day. As a rule of thumb, the bigger the battery, the bigger the array you want feeding it. If you are sizing up your battery, it is worth checking whether your solar can keep pace, and adding panels if not.
Get your sizing right and a battery shifts from a nice to have into a system that quietly pays itself off. Greenlight Solar can pull your usage data and model the payback on each size before you commit. Take a look at the home battery range or book a quote to get sized properly.
Frequently asked questions
How do I calculate what size battery I need?
Find your overnight and evening electricity use, not your total daily use, since solar covers your daytime load directly. Bill or meter data will show it. Then choose a battery that covers a normal night with a little headroom, without leaving large capacity uncycled.
Is a bigger battery always better?
No. A battery you cannot fill or fully use each day is wasted money, and under the 2026 tiered rebate you pay less subsidised rates on capacity above 14 kWh. Size to your real overnight demand rather than to the largest system on offer.
What size battery works with a 6.6 kW solar system?
A 6.6 kW array pairs well with a battery around 10 kWh for most smaller households. If you want 13 kWh or more, consider expanding your solar toward 8 to 10 kW so the battery can refill reliably, especially in winter.
Will a 10 kWh battery run my whole house overnight?
For a smaller or efficient home, usually yes. For a family home running air conditioning through the evening, or one with an EV, 10 kWh will often run out before morning and you will draw from the grid. Match the size to your overnight load.
Do I need a bigger battery if I have an EV?
Generally yes, or a smarter charging setup. Charging an EV at home overnight can use as much power as the rest of the house combined, so households with an EV often look at 15 kWh or more, or a system that charges the car directly from solar during the day.
Why is 14 kWh considered the ideal size in 2026?
Because the federal rebate pays its full rate only on the first 14 kWh of usable capacity, then tapers sharply above that. Sizing around 14 kWh captures the strongest value per kilowatt hour before the next rebate step down on 1 January 2027.
