Batteries

Adding a battery to your rooftop solar system allows you to store any excess energy generated and then use it when the sun isn’t shining.

Batteries can be used to store energy generated from solar panels for later use.

Learn about the costs and benefits of adding a battery to your existing or planned rooftop solar system, to decide if it's the right option for your home or business.

Reasons to get a battery

A battery can:

  • store energy generated by your solar system for later use
  • provide electricity during power outages, if configured to do so
  • reduce electricity bills.

For many homes and small businesses, the cost of a battery may outweigh the financial benefits. Keep reading to learn more about what a battery can do and decide if it is the right choice for you.

A solar retailer or installer might suggest adding a battery to your rooftop solar system.

Ask them why they think it is a good idea and decide if a battery suits your needs and budget.

Storing excess energy

If you don’t have a battery and your rooftop solar system generates more electricity than is being used at any point in time, the excess will usually be exported to the grid. If your system has an export limit, excess electricity above the limit cannot be exported to the grid and will be wasted (often referred to as ‘curtailed’).

A battery will store the excess energy for later use. This can:

  • reduce the need to buy electricity from your retailer
  • reduce curtailment of your solar export if you have an export limit
  • reduce your reliance on the grid
  • increase your solar self-consumption (solar electricity used within your property)
  • increase your self-sufficiency (the proportion of your home or business electricity usage supplied by your solar and battery system).

Of course, a battery is only useful if you have 'excess’ electricity to store, so it doesn't make sense to install one for a small rooftop solar system.

Back-up supply in a power outage

Very few grid-connected solar-only systems provide back-up power during a power outage (blackout), because the inverter shuts down when it detects the outage. This shutdown is called ‘anti-islanding’ and happens automatically for safety reasons—to stop solar generation going into the grid when technicians may be working on it.

A battery can provide back-up power during an outage, but it must be configured to do so. Not all battery systems can do this.

There are 2 common solar and battery set-ups, which operate differently during an outage:

  1. With some systems, the solar inverter shuts down and the battery supplies electricity to run appliances. Once the battery is discharged, there is no more electricity until the system is reconnected to the grid.
  2. An ‘islandable’ solar and battery system will disconnect from the grid during an outage and continue to power your home or business. When it is sunny, the solar will also charge the battery system. This is a more expensive option but may provide electricity over multiple days if electricity use is carefully managed.

There are also a few solar-only inverters which are islandable allowing you to use solar generation during a blackout. However, without a battery to store energy they will only provide power when the sun is shining.

Depending upon the type, size and set-up of your battery, it may provide back-up to the whole property or only to selected circuits — for example ‘essential’ circuits such as lighting, refrigeration or water pumping — or to a single phase if the property has a 3-phase supply.

During a power outage, you may need to manage your electricity use to make the most from the energy stored in your battery and avoid overloading the battery inverter.

If you live in an area with frequent grid outages, a battery may be a useful purchase. Find a solar retailer or installer with experience designing systems for resilience and talk to them about your needs.

Financial benefits

A battery can reduce your electricity bill by:

  • reducing the electricity you buy from your electricity retailer
  • taking advantage of time of use tariffs
  • reducing peak demand
  • letting you participate in a virtual power plant
  • reducing curtailment (likely to be only a small saving).

The savings you make from a battery may not be enough to pay for the cost of the battery within its warrantied lifetime.

Learn more about the costs and benefits of batteries and how they can reduce your bills.

Weigh the costs and benefits of adding a battery with SunSPOT.

SunSPOT doesn’t account for curtailment or participation in a virtual power plant.

Battery technology and safety

Types of battery

Most household and business batteries are lithium-ion batteries.

Lithium-ion batteries have high performance, long lifetimes and low maintenance needs. They have almost entirely replaced older, lead-acid batteries in the market.

Other emerging technologies such as solid state, iron, sodium-ion and flow batteries are currently under development and may become more commonplace in future years.

Battery size

Battery capacity is the amount of energy which can be stored in a battery, measured in kilowatt-hours (kWh).

  • Household batteries have a typical capacity of 4 kWh to 14 kWh
  • Commercial batteries can have capacity up to 100 kWh or more

Because batteries cannot be completely discharged (or emptied), the usable capacity is less than the actual capacity. For lithium-ion batteries, the difference between usable and actual capacity is small (5% to 10%).

When a battery is charged and discharged, a small amount of energy is lost. This is called efficiency loss. For a lithium-ion battery, this is typically about 10% of the stored energy.

The rated power output is the amount of electrical power the battery can output, measured in kilowatts (kW). It is also called the maximum discharge rate.

This limits the number and size of appliances that can be powered by the battery at any time without drawing electricity from the grid.

You can think of a battery as a water tank, filling up with energy instead of water.

The capacity (measured in kWh) of the battery is the amount of energy it can hold, like the capacity of a tank (litres) is the amount of water it can hold.

The power output (measured in kW) is how fast the energy can flow out of (or discharge from) the battery. You can think of this like the flow rate (litres per second) of water from a tank.

If you have appliances which are all turned on at the same time, or a large load like a spa bath which draws more power than your battery’s maximum power output, your battery will be unable to provide all the power. As a result, your property will also draw power from the electricity grid.

If you use large amounts of electricity in the morning and evening when there is no solar electricity being generated, you will need a battery with a large capacity to avoid drawing electricity from the grid during these times.

Talk to your solar retailer or installer to help determine the right battery size for you.

Battery safety, lifetime and quality

When buying a battery, you should:

  1. Use the Clean Energy Council's approved battery list to check that it has been independently tested and meets electrical safety, product and quality standards. Search for batteries on the approved list.
  2. Consider the expected lifetime of the battery which can be measured by:
    • years
    • cycles (the number of times it is charged and discharged)
    • throughput (the amount of energy stored and delivered)
    • a combination of these factors (years, cycles, throughputs).
  3. Check the warranty conditions.
  4. Read online product reviews.
  5. Look for manufacturers and products with positive reviews and technical support based in Australia.

Ask your solar retailer or installer:

  • how long the battery manufacturer has been in the industry and whether they have a local or Australian office
  • if the battery complies with standards for safety and performance
  • whether the manufacturer can identify the battery by batch number if there is a fault
  • if the battery supplier has Australian recall insurance or can prove that they can afford to conduct a recall if needed
  • which warranties are the responsibility of the installer or the manufacturer.

You can choose to purchase a battery from a New Energy Tech approved seller who has signed up to the New Energy Tech Consumer Code standards that cover sales, quotes, contracts, installation, warranties, and support. Look for an approved seller.

Your solar retailer or installer should be able to help you navigate these considerations.

Installation

There are specific rules about where a battery can be installed, including:

  • restrictions on the wall material the battery is attached to
  • minimum distances from habitable spaces, doors and windows.

To make sure your home or business battery is safely installed ensure your installer is registered/licensed and is accredited for battery installation.

Integrating solar and battery

The way a battery is integrated with your solar system is described as AC coupling or DC coupling.

If you are installing solar and a battery at the same time, either AC coupling or DC coupling can be used. If you want to add a battery to an existing solar system, AC coupling is the usual arrangement.

Your solar retailer or installer will choose the best arrangement to suit your needs.

AC coupling

AC coupling involves 2 types of inverter.

The solar inverter (a string inverter or multiple microinverters) converts DC electricity from the panels into AC electricity, to power your appliances or be exported to the grid.

A separate battery inverter converts this AC electricity back to DC electricity to charge the battery. It also converts DC electricity from the battery to AC electricity to power appliances or be exported to the grid. Some battery brands and models have the battery inverter built in.

DC coupling

DC coupling uses a single hybrid inverter for the solar and battery.

DC electricity from the solar panels can charge the battery directly.

The inverter converts DC electricity from the panels or battery to AC electricity which can power your appliances or be exported to the grid.

Battery management and set-up

Most batteries have a battery management system which controls how the battery charges and discharges.

The battery management system can be set-up in different ways to provide back-up power, give the best bill savings, or to participate in a virtual power plant.

Your solar retailer or installer will usually configure the battery to suit your needs. This may include entering details of your electricity pricing plan.

When your electricity plan changes in the future, the battery may need to be re-configured. Talk to your solar retailer or installer about who will do this.

The battery or battery inverter may need an internet connection to enable virtual power plant participation or system monitoring.