The rate is agreed by you and AGL when you take up a plan from AGL.
Some plans have fixed rates and others have variable rates.
I suggest you do some research into your usage and what you expect to get out of your new solar panel system.
Just remember if you get a 5kw Inverter the maximum output from your system will be 5kw even though you have say 6.6 Kw of panels. ( The extra power is available on overcast days).
In summer on a full sunlight day you may produce say 40Kw a day while winter you may produce only 10kw or less.
Also your house will consume your solar output first and only the excess power you generate will be exported to the grid.
If you are a HIGH consumer any solar input will be of benefit, but it will take you longer to offset the cost of the install.
At the moment I think that AGL's best solar rate is around 18 cents but with this comes a higher buy from the grid price and supply price and the fact that you do not get any discount on the grid price.
In my case we are not big power users (about 6kw a day) so actually moved to AGL because of the Solar rate they had on offer at the time (TWICE what I was getting).
Last billing period I used 463kw but generated 2363 kw for the 90 day period with a net result that my bill was a Credit of about $193 dollars, my previous was an $111 credit, and the one prior was a payment of $71.
Prior to installing solar my average bill was around the $350 mark.
But in addition to installing Solar also had a new hot water system installed (for free) and this is set to heat the water during the day (thus using solar) this alone is saving about $40 off my bill each quarter.
The cost of the solar install was $3,299 and in the year that we have had it, we have saved $1633 or just over half of the cost of the solar.
Something that I consider to be a good cost saving investment.