Powerwall 2 is backup power compatible and is available for an additional $399.
In the event of a power outage you will be able to power low energy appliances. Backup power will require modifications to your switchboard which is not included in the cost of a standard installation.
The availability of backup power during a power outage is dependent on having excess power stored in your battery to use at the time of any power outage. Speak with an AGL battery expert about whether backup power is right for you.