How much battery capacity do you need?


When people start looking at energy battery storage one thing they often ask is I have “x” kW solar on my roof at the moment, how much battery storage do I need?

This isn’t an easy question to answer and there definitely isn’t a one size fits all answer. If someone tries to sell you a battery without asking about your lifestyle and what your needs are then walk away. Chances are they are just trying to sell you their battery and not really working towards what is best for you. The cheapest battery upfront may turn out to be more expensive in the long run if the battery technology isn’t matched to your needs.

The amount of storage isn’t really related to the amount of solar you have on your roof (although it is definitely part of the equation) but it is more to do with how much power you use on a daily basis, how long you want backup power for and what appliances you want to operate from the battery.

The first step to working out the size of a battery storage component of your energy storage system is to look at your current electricity bills and find out what your current average daily usage is in kWh. You need to look at this over a whole year because usage will vary between different seasons. In the winter you may use more electricity for heating or clothes drying but in summer you may use an air conditioner.

Some households that are very energy efficient with only 1 or 2 people may only use 10 kWh of power a day or even less, however families with children that don’t make a particular effort to be energy efficient and have energy hungry appliances may use 30 kWh of energy a day or even more.  The average energy usage for a family of 4 is usually somewhere between 18 and 25 kWh a day. We have a usage calculator that may help you and if you would like a copy please do not hesitate to contact us.

The next question to ask is how much of that power is used during the day and how much at night. Power that is used during the day can come directly from solar panels but power that is used at night needs to come from batteries. There will be a background amount of energy that is used all the time like fridges and freezers but things like cooking equipment, TV’s and computers may only be used at certain times of the day. Very roughly if people are home during the day you can split your daily power usage in ½ between day and night usage but if no-one is home during the day a much higher percentage of that power will be used at night.

You then need to look at what equipment you have in your household and how much power they use. Things like air conditioners and clothes dryers can be very energy hungry and would require much more battery storage if you use them. Other things that use a lot of energy may be electric ovens or building tools like welders. Water pumps for tanks, kettles and fridges may require a high surge power to start up, although they may not run for a long period of time. Not all battery technologies or inverters can handle this surge power, particularly some lithium ionbatteries, so this needs to be taken into account, if you want a battery for anything other than load shifting.

When deciding on the size of your energy storage battery you need to ask what you want the battery to do.

  • Some people only want to do a little load shifting to use some of the solar generated during the day by their solar panels to use at night.
  • Some people want to be completely energy independent and want to use as little grid power as possible, while still staying connected to the grid as a backup source.
  • Some people want to “stick it up the power companies” and go completely off grid.
  • Some people want to protect themselves from power blackouts. This can be for a couple of hours or a couple of days.
  • Some people want to protect themselves from power blackouts but only enough to run certain appliances like fridges and lights rather than the complete house.
  • Some people have medical conditions that require a guaranteed supply of power.
  • Some people want to protect the environment from coal powered energy generation so want to have the most environmentally safe battery that provides power for as long as possible.

Each of these scenarios will require a different battery size and although all energy storage batteries can operate to shift load, not all battery technologies can operate in the case of a power blackout or to go off grid. The more you want your solar PV and battery to do the larger amount of solar PV and storage you will need.

Particularly when building a new house, you can choose how many electric appliances to incorporate. If you want to become energy independent it may be better to put in a larger solar PV array and more storage and use electric heating instead of gas, particularly with the improving efficiency of reverse cycle air conditioners for heating and cooling and heat pumps for water heating.

If you have an electric vehicle or are thinking of getting one in the future, you will need more solar PV and / or battery storage depending on when you want to recharge your vehicles battery. Of course it would be best if an electric vehicle is charged directly from solar panels during the day rather than going from one battery to another.

Once you have considered and worked out answers to as many of these considerations as possible then you need to find an installer that will offer you a range of battery technologies and will discuss your unique requirements to make sure you get the most suitable battery technology and the best battery size for your needs.

The upfront cost of your battery shouldn’t be the only consideration. You must make sure that the battery and inverter will do what you want them to do. If you only want to do a little load shifting this isn’t so critical (although there are other safety and recycling issues that you should consider) but if you want power when the grid goes down and want to use appliances with large surge draws to start up, the battery technology you use is much more critical as not all battery technologies will allow this.

If you want to discuss the best size of energy storage battery for your needs or would like to be referred on to one of our installers in your area please do not hesitate to contact us on 1300 474 333.or

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