Energy Star® rated window air conditioners use about 10% less energy than non-rated A/C room units. In addition to having great features (like timers and temperature control), they can save you $60 during the machine’s lifetime. Look for the yellow label on appliances in the hardware store, or check out different models on the EPA’s Energy Star® website:
http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=AC