We have a/c, not ducted though. So not sure if this will help. Like Nelle has said, running your a/c at 25 degrees is best. Have a look at your energy's suppliers website. They'll have handy hints for using a/c's.

1. How big is your house?

4 bedroom 2 bathroom and 2 living areas. We have split system air cons in every room of the house.

2. How many people live in your house?

5

3. What is your electricity bill during winter months?

around $300

4. What is your electricity bill during summer months?

$700

5. Do you have any big electricity "suckers" (pool, spa, excessive use of dryer etc)

We have a pool which is on a cheaper tariff. It only costs around $40 per quarter to run.

6. How often do you use your air conditioning?

None at all during winter.
During summer, the girls air con goes on when they have a sleep, and the split system in one of the lounge room is one most of the day.
We have all brm air cons running at night.