Still doesn’t make sense…an airconditioner has to ‘fight’ heat transferred from outside the airconditioned room. The amount of energy absorbed by the room will be the same irrespective of whether the room is airconditioned or not (in fact, the amount of energy absorbed and retained within a house will be less than that being fought by the airconditioner in longer periods…otherwise a room will continue to get hotter and hotter and never cool until the outside temperature decreases/night time…in reality, a room will heat up until such time the amount of heat being radiated will equal the amount being absorbed). The factors which will affect heat absorption are things like roof and building colour, insulation, construction materials, curtains etc.
If the room heats up, it is still the same heat that the house would otherwise have absorbed and the airconditioner cools. There becomes a point where the temperature within the dwelling won’t change much through heat being absorbed and retained (the heat equilibrium outlined above)…depending on the type and insulation in the house, this could be less to an hour to several hours. Anytime after this there will be considerable energy savings. Before this point, any saving is likely to be minuscule and due to air leakage within most (all?) Australian homes, the cost to run the airconditioner will be greater as cool/hot air will be continually lost and replaced with external air, in effect, increasing the amount of work the airconditioner would otherwise have to do…
The other thing to consider is many businesses/office complexes will have night airconditioning cycles where the airconditoner is either shutdown or runs at a more efficient setting (either at higher/lower temperatures or at lower intensity). I have worked in a large office building where, outside normal business hours, the airconditioning shut off completely and one had to contact security should one need airconditioning at such times (e.g. running meetings with external clients on a weekend). If it was cheaper to run the aircon all the time, business would not be doing the above.
The other thing to consider is coolness/warmth comes from air temperature and radiated sources. A chair/bed or other furniture is unlikely to have a high embodied energy and radiate out much heat to make one feel warm.
Also, the higher the differential temperature between two bodies, the quicker the energy loss. If one runs an airconditioner, it is highly likely that the airconditioned room temperature would be significantly greater than a non-airconditioned room. This greater temperature differential would increase running costs and would be better to turn off the aircon when not in use. The only way this factor would not come into play is if the insulation was near perfect, and there were next to zero losses from a airconditioned room. In reality, this is impossible as there will always be come loss unless one lives deep under the ground or in a vacuum.