And air conditioner is in the most simple terms a heat transfer device (or heat pump). It takes a certain amount of electricity per BTU of heat transferred. It doesn't matter to the heat pump (A/C) whether you are transferring heat from a 95 degree home or a 75 degree home. You are simply using the electricity to transfer heat from inside your home to outside. Turning the thermostat down to 65 doesn't make the house cool down any faster (much like turning the heat up to 95 doesn't make it heat up faster).
When it is hotter outside than inside your home, heat is transferred (free of charge) from outside to inside until both reach the same temperature. You have to put energy into your A/C in order to reverse this effect. The amount of thermal mass (or stuff you have in your home) does effect how long it takes for this balancing act to occur. If you have lots of furniture, it will take more BTUs to heat up your house. You have to realize that it also took the same number of BTUs to cool off that furniture and stuff in the first place. In other words, the effect cancels out. I think that this is biggest source of confusion. When I was growing up (before engineering school), I too was also told to leave the thermostat alone because you didn't want to let all that stuff heat up. The only disadvantage to letting your stuff get warm is that it will take longer to cool it all back down again. But remember that it took longer for it to heat up as well. BTUs in equal BTUs out.
So if you leave your thermostat set at a fixed temperature, it will have to work to maintain the status quo (or the difference between indoor and outdoor temperature). As the outside world attempts to heat your home, you use electricity to balance out the heating effect.
By turning off the A/C, you stop fighting the battle to maintain status quo. Your house will heat up (and cool down) as the outside temperature rises and falls. And it won't cost you anything to allow that to happen.
When you turn the A/C back on, it will transfer BTUs out of your home until the status quo is once again established. Herein lies another difficulty. If it is gosh awful hot outside when you attempt this, and if your A/C is sized correctly, it might not be possible for your A/C to drop the temperature inside your home as quickly as you would like. But it is not taking any more energy to transfer those BTUs than it would have if you had left the A/C on all the time.
What I am trying to describe is entropy, the effect that tries to bring everything in the universe into balance. Given enough time, everything in the universe will eventually reach equilibrium. There's no way around it (in the very long term).
Last winter, there was a similar thread on the DIS about heating your home in the winter. Since heating and cooling are actually the same (just swap inside and outside in my previous descriptions), the analogy holds.
Think of your home as a pan of water on the stove. Which takes more energy?
A. Leave the pan of water simmering so that you can easily bring it to a boil by simply turning up the burner slightly.
B. Leave the burner off until you want to boil the water.
Although answer A would provide you with nearly instant boiling water, the cost to keep the stove turned on day and night to keep the water warm is obviously more than case B.
So keeping your house at a constant indoor temperature is going to cost more than to let the house warm up while you are out. But you'll have to explain that to my wife when she arrives home this evening to the little note that I left on the kitchen counter explaining that I turned off the A/C when I left home this morning because nobody was going to be home. My little part to reduce global warming.
