During a blackout do you need to use the phone if you are on the NBN and use the internet to keep track of things.
Then consider a UPS
An uninterruptible power supply is an electrical device that provides emergency power to a load when the input power source or mains power fails.
When the power goes out the UPS can then power the modem and phone.
Depending on what is plugged in and which UPS you buy , you can then get 3 -4 hours or more.
You can buy a UPS from Allneeds Computers , 342 King William St , Adelaide Ph:
(08) 8211 8661 – email@example.com
They start at just over the $100 mark