WATER SAVING TIP: Wash your laundry with cold water whenever possible. To save water, try to wash full loads or, if you must wash a partial load, reduce the level of water appropriately. Hot water heating accounts for about 90 percent of the energy your machine uses to wash clothes — only 10 percent goes to electricity used by the washer motor. Depending on the clothes and local water quality (hardness), many homeowners can effectively do laundry exclusively with cold water, using cold water laundry detergents. Switching to cold water can save the average household more than $40 annually (with an electric water heater) and more than $30 annually (with a gas water heater). Washing full loads can save you more than 3,400 gallons of water each year.
-excerpt from EnergyStar.gov