The key to keeping your house cool in summer is to ensure it is well insulated and draft proofed. The most efficient type of air conditioning is with a heat pump. Look for the ENERGY STAR logo when purchasing an air conditioner.
Room air conditioners have a minimum Energy Efficiency Ratio (EER) of 10.7 to qualify as ENERGY STAR.
Central air conditioners have a Seasonal Energy Efficiency Ratio (SEER); with the minimum SEER of 14 to qualify for Energy Star.
Open the windows in the evening to cool off house; well-insulated homes will keep the cool air inside when blinds are shut during the day.