One of the most common questions we hear from homeowners is: “What size battery should I get?”
It’s also one of the most misunderstood parts of buying a home battery.
Many people come to us already thinking they need a particular battery size. They may have heard that a friend has a 20 kWh battery, seen a particular model advertised online or simply assumed that bigger must be better.
But the right battery size is different for every home. A battery should be sized around how your household actually uses electricity and what you want the system to achieve.
Bigger isn't always better
It can be tempting to simply install the largest battery you can afford. But that isn’t always the best use of your money.
If your battery is much larger than your household needs, you may rarely use all of its available capacity.
Equally, if your battery is too small, it may regularly run out of energy early in the evening, leaving you to purchase electricity from the grid for the rest of the night.
The objective is to find the right balance. A properly sized battery should work effectively with your solar generation, household consumption and electricity tariff.
Start with how much electricity you use
Your electricity consumption is one of the most important starting points, but total daily consumption doesn’t tell the whole story.
Two homes could both use 25 kWh of electricity per day and still need very different battery systems. Why?
Because when that electricity is used matters.
For example, a household that uses most of its electricity during daylight hours may already be using a large proportion of its solar directly.
Another household may be largely empty during the day but use substantial electricity from late afternoon through the evening.
That second household may benefit more from additional battery capacity because there is more evening consumption to cover.
Evening and overnight usage matters
For most homes, the key question is:
How much electricity do you use after solar production drops away?
That may include:
- air conditioning;
- cooking;
- lighting;
- televisions and appliances;
- pool equipment;
- electric hot water;
- EV charging; and
- general household electricity consumption.
If your objective is to substantially reduce grid electricity use, the battery ideally needs enough usable capacity to cover a meaningful portion of that evening and overnight demand.
Your solar system matters too
A large battery is only useful if you have a sensible strategy for charging it.
If you have a relatively small solar system but install a very large battery, there may be many days where your solar generation simply isn’t enough to fully charge it. Sometimes the better investment is not just a larger battery.
It may be:
more solar + an appropriately sized battery.
Available roof space can therefore be an important part of battery sizing.
Where suitable roof space exists, increasing solar generation may improve the overall performance of the home energy system more than simply adding more battery capacity.
Existing solar systems need to be assessed carefully
If you’re adding a battery to an existing solar system, there are additional considerations.
We look at things such as:
- the size of the existing solar system;
- how much electricity it actually produces;
- the age and type of inverter;
- roof orientation;
- shading;
- how much solar is currently being exported; and
- whether additional solar could or should be added.
A homeowner with a large amount of unused solar generation may have a very different battery requirement from someone whose existing solar system is already struggling to meet daytime consumption.
Your electricity plan can change the answer
Battery sizing isn’t only about solar. Your electricity tariff can also influence the recommendation.
If your electricity plan offers low-cost or very cheap electricity during certain periods, your battery may be able to charge from the grid during those periods.
That can make a larger battery more useful, particularly during winter or poor weather when solar generation is lower.
A battery that can be strategically charged when electricity is cheap and discharged when electricity is expensive can operate very differently from a battery that relies primarily on excess solar.
That’s why we prefer to look at the whole energy picture rather than just daily electricity consumption.
Think about where your electricity usage is heading
A battery should also be designed with the future in mind.
Many Australian households are gradually using more electricity. You may currently have gas hot water, gas cooking or petrol vehicles, but that could change over the coming years.
Future electricity demand might include:
- an electric vehicle;
- a second electric vehicle;
- heat-pump hot water;
- induction cooking;
- replacing gas heating;
- additional air conditioning;
- a pool or spa; or
- renovations or an extension.
If these changes are likely in the near future, they should be considered when sizing both your solar system and your battery.
The aim isn’t to dramatically oversize the system “just in case”. It is to make sensible allowances for changes you already know are likely to happen.
What are you actually trying to achieve?
This is perhaps the most important question of all. Different homeowners have different priorities.
For some, the objective is simply: Reduce my electricity bill as much as reasonably possible.
For others it might be:
I want to minimise how much electricity I buy from the grid.
Others might prioritise:
I want strong backup capability during outages.
And some homeowners are preparing for significant future electricity consumption. These goals can lead to different battery recommendations even for two otherwise similar homes.
A simple example
Imagine two households that both use around 30 kWh of electricity each day.
Home A uses most of that electricity during daylight hours and has a large solar system.
Home B is empty during the day but uses large amounts of electricity between 4 pm and midnight.
Home B may benefit from considerably more battery capacity because much more of its consumption occurs after solar generation has fallen away.
Simply knowing that both homes use 30 kWh per day doesn’t give us enough information. The usage pattern matters.
Why we don't start with the battery
At AG Solar, we don’t believe the first question should be:
“Which battery do you want?”
The better questions are:
- How much electricity do you use?
- When do you use it?
- How much solar can your property generate?
- What electricity tariff are you on?
- And what are you actually trying to achieve?
Once we understand those things, we can recommend an appropriate combination of solar and battery capacity.
That may mean a smaller battery than you originally expected. Or it may mean a larger battery makes sense because of your evening usage, future electricity needs or electricity tariff.
The important thing is that there is a reason behind the recommendation
Get the system sizing right
There is no universal “best” battery size. The right answer is the battery that works properly with your property, your solar generation, your energy consumption and your goals.
At AG Solar, we design solar and battery systems around how your household actually uses energy rather than simply selling a standard package.
Based in Caringbah, AG Solar has extensive experience designing, installing and supporting solar and home battery systems throughout the Sutherland Shire, Southern Sydney, Wollongong and the Illawarra.
If you’re considering a battery and aren’t sure whether you need 10 kWh, 20 kWh, 30 kWh or something else entirely, speak with the AG Solar team.
We’ll help you understand the options and recommend a system based on what you’re actually trying to achieve.
