Yeah, I do the same thing with my car. I let it idle all night so I don't have to use extra gas in the morning to warm the engine and the cabin up. It uses less gasoline that way.

Pigeon, why on earth would you think that? It takes much less energy to recool the house than it does to maintain a constant temperature lower than outside. First, you turn the AC off for hours and hours, that uses 0 electricity. Then you turn it on, it runs on full for 20 or 30m or so to cool the room. My AC uses 600 watts on full power. Once it reaches cool temp, it maintains the temperature by running on low, which is 100 watts. However, it periodically jumps back up to 600 watts to recool the place, about 25% of the time. So, the average hourly usage is 225 watts. So, if you assume it takes 30m to cool down the room on full, it only makes sense to leave the AC on if you're going to be gone for an 90 minutes or less. Otherwise, you're saving money by turning it off (assuming you have no pets to keep cool).