Solar battery size: How many do you need?

Selecting the right battery size

Selecting the right battery size for your needs depends on:

• Your motive for adding a battery
• Your electricity use
• Your budget
• Your panel array size (new or existing)

What is your motive?

Most people buy a battery to try to eliminate the electricity they purchase from the grid in the hope of making savings. Some may want a battery to provide backup power, while for others it may be all about using solar energy to power their home.

If your motive is to make savings, you should still expect to buy some electricity from the grid at certain times of the year. Sizing a battery to effectively take you off the grid is not economical.

Electricity use

The average Australian household uses 16kWh of electricity per day. A typical household might use a third of their electricity (5.33 kWh) during daylight hours when solar panels are producing electricity, while the remaining two thirds (10.67 kWh) are purchased from the grid. If you were to use a battery to power the electricity you purchase from the grid, you would need a battery with a capacity of at least 10kWh.

How much are you prepared to pay for a battery?

If you are not prepared to pay over $10,000 for a battery storage solution, there aren’t many batteries available to you that will be a suitable size for your electricity use. Solar batteries cost roughly $1,000 per kWh, for a 10kWh battery, you’re looking at around $10,000.

If you are a low energy user, you could get a small battery less than 5kWh to serve your needs.

New system or adding to an existing system

Buying a battery is far more economical when you buy a new panel and battery system, rather than adding storage to your existing system. The steps to calculate your battery size also differ depending on whether you already have solar panels, or are looking to buy a solar and battery system.

How to calculate battery size

The battery size calculation is different depending on whether you are:

• Buying a new panel and battery system
• Or, already own solar panels

Solar and battery size calculator

Our solar panel and battery size calculator will tell you how many panels you need, and what size battery you need. All you need to know is your daily electricity usage and an estimate of when you use it and the calculator will do the rest.

For existing solar panel owners

Our solar battery storage calculator allows you to play around with different size batteries to see the effect each has on payback and savings. This is the best way to size a battery for existing solar owners, as the financials dictate what size you need based on your electricity usage habits.

Battery size for a 5kW system

Regardless, if you already have a 5kW system, or are looking to purchase one, you’ll likely need a battery with a capacity of at least 10kWh, more likely, up to 13.5 kWh. The exact battery size you need depends on how much electricity you typically consume during daylight hours and your purchase motives.

Why you likely need 13kWh battery for a 5kW system

A 5kW solar power system typically produces anywhere between 17 – 21 kWh per day depending on where you live in Australia. For most consumers, around two-thirds of a system’s production is fed back into the grid, which is around 13 kWh per day. This 13kWh could instead be used to store your solar power in a battery.

If you use closer to 50% of your power during daylight hours, around half the production will be sent back to the grid, meaning that you could charge a slightly smaller size battery, more like 10 kWh.

Suitable batteries for 5kW systems

Due to the popularity of system sizes around 5kW and 6.6kW, some of the best solar batteries are geared to serve systems of this size. The LG Chem and Tesla Powerwall II batteries are both suitable options for a 5kW system.