ENERGY STAR certified dishwashers are are 12% more energy efficient and 30% more water efficient than standard models. A standard-size ENERGY STAR certified dishwasher costs about $35 annually to run and will save about 3,870 gallons of water over its lifetime.
If you want to save even more water: scrape don't rinse. Pre-rinsing dishes before loading the dishwashers can use up to 20 gallons of water. Just scrape food off the dishes and load. ENERGY STAR certified dishwashers and today's detergents are designed to do the cleaning so you don't have to pre-rinse. If your dirty dishes are going to sit overnight, use your dishwasher's rinse feature. It uses a fraction of the water needed to hand rinse.
Also keep in mind, washing dishes by hand uses much more water than using a dishwasher. Using an ENERGY STAR certified dishwasher instead of hand washing will save you annually 7,000 gallons of water, $111 in utility costs, and 230 hours of your time.