How much of the green-house gas (CO2) will my system save?
How much CO2 is avoided depends on the conventional electricity it displaces and this varies by State/Territory and generation technology. In principle, solar creates no CO2 (or other emissions), other than during manufacture and transport.
The CO2 emissions associated with the manufacture and transportation is offset by the panels operation within the first 16-18 month of operations in Australia. So after that timeframe your solar system will be carbon neutral.