How Many Batteries Do You Need?
Good question. It depends on what you want them to achieve. Most people set out with the aim of reducing their grid imports to zero.
As you'll soon see this lofty aim will actually be very expensive for most homes.
Let's do a quick crash course on how much storage an ordinary Aussie household will need.
The average Australian home uses roughly 16kWh of electricity per day (Please read this if you are not completely happy with the difference between a kW and a kWh - it is super important).
Naturally, if you have a house that has a big air-con and pool pump etc, you'll use a lot more than that.
Assuming that most Australians work a 9-5 job and are not home during the daytime, they'll probably only use only around 30% of their electricity during the day, when the sun is up and their solar panels are producing electricity.
That means that they need to buy the other 70% from the grid.
How many batteries do you need to need to cover that 70%?
Sizing your system
In terms of system sizing - battery sizes are expressed as kilowatt-hours, or kWh. The Tesla Powerwall, for example, is a 6.4kWh battery.
If the average home uses 16kWh, 30% of this during the day and 70% at night, that works out to about 5kWh of daytime usage, and 11kWh of nighttime usage.
So, simple math would dictate that the average Aussie would need about 11kWh of battery storage to offset all their night usage.
If you are using Powerwalls, then you'd need 2 of them.
But there is no need to offset all nighttime your electricity with batteries. If you run the numbers, the best payback is from the first kWh - because it works the hardest, as you add subsequent kWh their payback gets longer. So don't bankrupt yourself looking to cover every last kWh of nighttime electricity with your batteries. Start with one battery pack and add more later if and when you decide it is necessary.
In fact, a modular option is the Enphase AC battery which can be added in 1.2kWh modules for about $2,000 per pop. Subsequent batteries are simply plugged in.
But the most important thing to consider when sizing batteries is that it is impossible to do without a detailed understanding of your energy usage profile. Which means that you need a good energy monitor on your home with good data as a first step. Then a good installer can use their software to show you exactly what the optimum number of batteries and solar panels is for your situation and what the payback will be. Then you are going into the investment with your eyes wide open, instead of it being a leap into the unknown.