Today’s modern central air conditioners are significantly more efficient than old models. Part of this is due to the federal push for Australian energy independence and a drive to reduce greenhouse gases. The federal Energy Star program mandates that the models you look at when upgrading an air conditioner have met MEPPS Minimum energy performance criteria, this is easily obtainable, you will conserve natural resources and lower atmosphere-fouling emissions.
This efficiency extends to the new refrigerant, R-410A, which provides impressive cooling power while not chipping away at the ozone layer. Modern equipment built to handle this newer coolant (as opposed to the old, discredited R-22/Freon) will deliver ample cool air with greater energy efficiency than your old system.