Today went from spring to summer in an instant. We went from 60 degree F temperatures to near 90 and to humidity in the 70% range. Setting the thermostat from 78 to 72 degrees is going to take a bite out of your wallet in a hurry, and the real problem isn't the temperature but the humidity. Even get out of the shower on a cold morning. Because the humidity is low, and the temperature is low and you're wet, the evaporation on your skin is going to cool you off. Problem is that there are two types of heat and one is call sensible heat.
Sensible heat is the temperature we read on a thermometer. If we boil water, the temperature is going to go up to 212 degrees (or 100C) until it boils and once it boils it will stay at that temperature until it has all boiled away, and then the temperature will go way up. The heat involved with that process is called latent heat. In air conditioning, this is all the heat that needs to be sucked out of all the furniture, wall, your plasma TV, etc before the temperature will go down. You'll also need to get rid of the humidity and that takes a lot of time.
Problem is, the blower is usually set to the highest setting which reduces the sensible heat, but doesn't take time to remove the latent heat or humidity. If you want to save money and stay comfortable, slow the blower down a notch and set the air-conditioner thermostat up to 77 or 78. The unit will run longer, but remove more humidity in the process. Because you can live with a higher temperature, you should be able to save money on your utility bills as long as you change the filter and have the system maintained regularly.
As always, a programmable thermostat, if programmed properly will help you save energy. This means never setting the temperature up or back more than 3 to 5 degrees. This is because the system will have to work harder to get the heat down then it would if it maintained. I had a bear of a time trying to convince my former tenants to leave their air conditioning on during the day on the hottest of same. The result was at five in the afternoon, the unit would never catch up and their electric bills would be sky high. They would complain that the air conditioners were at fault when the reality was they wouldn't listen. Air conditioners don't produce cool, they remove heat and this is like trying to bail water out of a boat with a teacup. The less water leaking the in or there already, the faster and more efficiently it will to keep the water from swamping the boat. This is the same thing with heat in a structure. I know I've beat this to death, but hopefully it'll sink in (pun intended). Maranatha!