What Size Home Battery Do I Actually Need?
If you're looking at home batteries, one of the first questions you'll probably ask is:
“How big should my battery be?”
You might see 10 kWh, 16 kWh, 24 kWh, 30 kWh or even 40+ kWh systems and assume that buying the biggest battery you can afford is automatically the best option.
It isn't quite that simple.
The right battery size depends on what you're trying to achieve, when your home uses electricity and where the energy to charge the battery is going to come from.
A battery designed to achieve the shortest possible financial payback can look very different from one designed to keep a home operating through an extended blackout.
That's why we don't start with battery size.
We start with the outcome.
If your goal is saving money, start with your load profile
If financial return is the priority, simply looking at your total daily electricity consumption isn't enough.
We want to know when you're actually using that electricity.
This is where interval data becomes important.
Rather than seeing that a household uses, for example, 30 kWh per day, interval data lets us see how that consumption is distributed throughout the day.
For many residential properties we assess, a large portion of consumption occurs outside the main solar-generation period — often from around the afternoon through to the following morning.
That's the energy a battery has the opportunity to cover.
If solar is generating while you're consuming electricity, that energy can generally go directly into the house.
It's the excess solar that would otherwise be exported, and the electricity you would later need to buy back from the grid, that becomes particularly relevant when sizing storage.
For a homeowner chasing the shortest practical payback, the objective generally isn't to install the largest battery possible.
It's to install the smallest cost-effective battery that can regularly be charged and discharged enough to deliver the required savings.
Bigger isn't automatically better
There's nothing inherently wrong with oversizing a battery.
The problem occurs when you pay for capacity you rarely use.
Imagine installing a 30 kWh battery but your normal energy profile only allows you to put around 10 kWh into it each day.
You've paid for another 20 kWh of storage capacity that is sitting there doing very little most of the time.
That additional capacity may still be valuable if you're deliberately keeping reserve energy for backup, expecting future loads or have another way of charging it.
But if your only objective is financial return, unused storage can extend the payback period.
The better question isn't:
“What's the biggest battery I can afford?”
It's:
“How much battery capacity can my home actually use?”
But don't only design for the home you have today
There is another side to this.
Going too small can also be shortsighted if you already know your electricity requirements are going to increase.
Maybe you're planning to buy an EV.
Maybe you're installing ducted air conditioning.
Maybe you're moving away from gas appliances.
Maybe your family circumstances are changing and you expect to consume significantly more electricity in the future.
Those loads can be modelled into the design.
We can look at what your property uses today and then create another scenario showing what the energy profile could look like after adding an EV, ducted AC or other major electrical loads.
That allows you to make a decision based on where your home is going rather than only where it is today.
This is particularly relevant under Australia's current Cheaper Home Batteries Program. Additional capacity added to an existing system is only eligible where that battery system has not previously received the program discount. In practical terms, if you've already received the federal battery discount, expanding the system later does not give you another opportunity to claim the same program discount on the additional capacity.
That doesn't mean you need to massively oversize the system today.
Many modern battery systems are modular and can be expanded later.
It simply means future requirements should be part of the conversation when the original system is being designed.
What if you want backup during a blackout?
If backup is the primary objective, battery sizing changes again.
You're no longer asking:
“What's the smallest battery that gives me the best financial return?”
You're asking:
“How much energy do I need to get through a blackout?”
If you only want to keep your fridge, lights, internet and a few essential circuits operating for several hours, your requirements may be relatively modest.
If you want to operate most of the house for one or several days, the required capacity can increase substantially.
You need to estimate:
What loads need to operate
How much energy those loads consume
How long you want them to operate without the grid
Whether solar is likely to be available to recharge the battery during the outage
For someone prioritising autonomy, having additional battery capacity can be exactly what they want.
That's why the "correct" battery size can't be determined until the purpose is understood.
Battery size isn't the same as battery power
There's another specification homeowners often overlook: inverter capacity.
The easiest way to understand this is to think of your battery like a rainwater tank.
The battery is the tank.
The inverter is the tap.
A large tank tells you how much water you can store.
The size of the tap determines how quickly you can get that water out.
If all you need is a slow trickle for a long period of time, a smaller tap may be perfectly adequate.
But if you suddenly need a large amount of water, the size of the tap becomes the limitation — regardless of how much water is sitting in the tank.
Your battery system works in a similar way.
You could have plenty of stored energy available, but if you want to run several large loads simultaneously, the inverter needs to be capable of delivering the required power.
So when someone asks us what size battery they need, we're really looking at two separate questions:
How much energy do you need to store?
and
How much power do you need to deliver at any one time?
Both matter.
Your solar system also needs to be considered
Installing a large battery doesn't automatically mean your solar system can fill it.
Let's say you install a large battery but your existing solar system produces relatively little excess energy after supplying the home.
The battery may never reach anywhere near its full capacity from solar alone.
That doesn't necessarily make the design wrong.
It simply means we need to understand where the remaining energy is coming from.
Some homeowners may deliberately charge their batteries from the grid during very low-cost off-peak periods and then use that stored electricity when grid rates are higher.
Others may have electricity plans that provide very cheap or even free electricity during certain periods of the day.
In those situations, the grid itself can become another source used to charge the battery.
So the relationship isn't simply:
Big solar = big battery.
The actual question is:
Where will the energy come from to fill the battery, and when will you use it?
There isn't one correct battery size for every home
Two houses using exactly the same amount of electricity each day could legitimately require completely different battery systems.
One homeowner may want the fastest financial return.
Another may want to comfortably run their home overnight.
Another may be preparing for an EV and ducted air conditioning.
Another may want enough stored energy to remain operational during an extended grid outage.
The daily electricity bill alone doesn't tell us which system is right.
We need to understand the household's interval data, solar production, current loads, future loads, electricity tariff and — most importantly — what the homeowner wants the system to achieve.
That's why we believe a battery should be designed and modelled for a purpose.
Don't start by asking:
“Should I buy a 16 kWh or 30 kWh battery?”
Start by asking:
“What do I want my battery to do?”
Once that answer is clear, the right size becomes much easier to determine.