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What size battery do I need?

What size battery do I need?

Battery storage is becoming increasingly popular amongst Australian homeowners.  Rising electricity prices combined with falling feed-in tariffs are prompting many people to wonder whether they would be better off storing their surplus solar rather than exporting it to the grid.

And they’d be right to wonder!

With the right size battery combined with the right size solar panels array, it is possible to get to zero-dollar electricity bills and be virtually 100% energy self-sufficient.

What size battery? The quick answer

The size battery you are most likely to need is between 10kWh and 14kWh.  It is the most popular size battery for grid-connected Australian homes and ideally suited to work with solar panels systems between 6.6kW and 10kW.  Most leading brands provide competitive battery products in this size category – so you get more choice too!

If you want to go off-grid, you’d need a battery bank that’s at least three times bigger and more panels as well.  The cost of an off-grid residential solar systems is at least 3-4 times more than a grid-connect solar system.  Most people who install off-grid systems do so because their property doesn’t have an existing grid connection and the cost of getting connected would run into the tens of thousands of dollars.

This post focuses on grid-connect batteries and covers:

  • How battery size is calculated
  • Reasons for wanting a battery
  • Working out your electricity usage
  • Size of your solar panels system. Why it’s important.
  • Budget and rebates


How battery size is calculated

To answer the question, “What size battery do I need?”, it’s important to firstly understand the terminology used to size a battery.

Battery sizes are expressed in kilowatt hours (kWh); this is a measure of energy stored in the battery. In contrast, solar panels systems are measured in terms of kilowatts (kW), a measure of the power that the system can generate.  Your electricity bill, which measures energy usage, states the number of kilowatt hours you use each day, or month.

Terminology used for battery sizing, solar panels and electricity bills

 BatterySolar panels systemYour electricity bill
Term used Kilowatt hours (kWh)Kilowatt hoursKilowatt hours
What this meansAmount of energy that can be storedAmount of power that can be generatedAmount of energy that is used

The average Australian home uses around 16kWh of electricity a day. Unless you are mostly at home, or you’ve installed Wi-Fi enabled smart electric appliances, chances are you are using around 30% of electricity during the day and 70% at night.

Doing the maths, that means during the day you are using:

16kWh x 0.03 = 4.8kWh of electricity

During the evening and night, you’re using:

16kWh x 0.07 = 11.2kWh

With this usage profile, you would need a battery that can store 11kWh of electricity.  But if you’re planning to use more electricity in your home in the future – for example for charging an electric vehicle, running an electric hot water heat pump or electric pool pump – you may need a bigger battery.

The SolarEdge battery is the ideal size battery for many homes, with 10 kWh usable electricity and the option to add more batteries if required. It requires at least 5 or 6kW of solar panels to power it – ideally 10kW.  The Alpha-ESS battery is another great option, with 13kWh storage size and is priced very competitively.

What are your reasons for wanting a battery?

There are five main reasons why homeowners invest in a battery:

  1. Zero-dollar electricity bills
  2. Protection from power outages
  3. Join a Virtual Power Plant
  4. Greater energy independence
  5. Reduce CO2 emissions

If you’re looking eliminate your electricity bills, you’ll need a big battery – most likely 10-14kWh.  Most people use more electricity at night, than in the daytime. With a battery, you can use the free solar electricity in your battery to power your home through the night.  Depending on your electricity usage, a 10kWh or 14kWh battery means you’ll be buying a lot less grid power and potentially even getting bills in credit!

Protection from power outages is the second reason why people invest in home battery storage.  No-one likes to lose electricity to their home, but for some households guaranteed power is essential.  For example, people running businesses from home, those relying on medical equipment, properties in bushfire hotspots that need be to pump water from a dam.  Depending on what systems you need to run, you may need a big battery, maybe even two. If this is your situation, spending several thousand on battery storage is like a comparatively cheap form of insurance. But beware, not all batteries have blackout protection.  Get in touch if this is important to you.

On the other hand, you may be wanting to get into energy trading and keen to join a Virtual Power Plant, or VPP.  VPPs are networks of homes with solar batteries connected using smart software. Still relatively new, VPPs are an exciting technological solution for making the national electricity grid more resilient and a lot greener. To take part in a VPP program, you need to have solar panels and a solar battery installed. Some state governments [link to Vic VPP  post] are rolling out VPP programs which reward households for their surplus solar and provide other incentives. Talk to us if this is of interest!

Becoming more energy independent is the fourth reason why many households get battery storage. Reducing reliance on the big electricity retailers appeals to many. With 6kW or even 10kW solar panels system teamed with a 14kWh battery, your household will be better protected from electricity price hikes and less exposed to changes in government energy policy.

Finally, many people want a battery to reduce their household carbon emissions.  By using clean solar energy to power their home day and night, households can cut back on grid imports.  Given that around three-quarters of electricity in the national grid is from non-renewable sources (like coal-fired power stations), using less grid power is better for the environment.

Thinking through the reasons why you want a battery is important. You may be wanting a battery for a combination of reasons – for example bill reduction, blackout protection and to join a VPP.  By considering all your requirements, and the relative importance of each, you have a great starting point for answering the question: “What size battery do I need?”

Working out your electricity usage

The easiest way to check your electricity usage is to look at your bill.  There is often an average daily usage figure in kilowatt hours.  Dig out your bills for the last 12 months to work out what your average daily electricity usage is over a year.

Next, you need to work out how much of that daily usage is when the sun’s out, and how much is in the evening and a night.  A good solar monitoring system can do that for you, or chat to one of our solar experts. Your aim is to work out what percentage of your usage is during solar-producing hours, and how much is used at other times.

Size of your solar panels system. Why it’s important.

We’ll start off by saying that if you don’t have at least 5kW of solar panels installed, there’s little point adding a battery.  Most residential batteries need at least 5kw or 6kW of solar panels to charge the battery. And if you’re looking to the future (i.e., all-electric home, EV charging and energy trading , 10kW of solar panels is going to be much more suitable.

It’s worth knowing that in some states you won’t even qualify for a battery rebate unless you have a minimum of 5kW of solar panels installed.

Budget and rebates

A lot of people are put off batteries because they think they’re too expensive.  But battery prices are coming down as government incentives make the market more attractive to manufacturers and competition intensifies.

Battery prices used to be around $1,000 per kilowatt hour of battery storage (including installation and GST).  So, a 10kWh battery would be priced at around $10,000.  But now, new batteries coming on the market are priced below that level. A great example is the Alpha-ESS battery which is around $700/kWh of storage.

The upfront cost of a battery can also be reduced significantly if you live in a part of Australia where your state or territory government provides a battery rebate.  These rebates can knock thousands off the cost of a battery and are well worth finding out about!

Also, if you choose an approved battery through a Virtual Power Plant program you may also be eligible for a further upfront discount on the battery price, as well as ongoing credits on your electricity bill in return for sharing your stored energy with the VPP.

All up, there are a lot of incentives that make it more affordable for Aussie homeowners to install home battery storage.  Combined with the great prices from SolarRun, we reckon you’ll be pleasantly surprised by the affordability of installing a battery on your home.

Get in touch for an obligation-free quote

Get in touch with us today for expert advice and an obligation-free battery quote.  We’ll can also advise you on the battery subsidies and incentives that may be available in your state. With government policies changing all the time, it pays to get in quick whilst the battery rebates are at a high level.

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