The efficiency of a dishwasher is measured by a term called the energy factor. It is somewhat similar to the miles per gallon for a car, but in this case the measure is cycles per kilowatt-hour of electricity. The minimum allowed energy factor rating for standard capacity dishwashers is 0.46. About 80 percent of the total energy used by dishwashers goes towards heating the water. So the best way to improve the efficiency of a dishwasher is to reduce the amount of water needed to clean the dishes. 
Federal law requires that EnergyGuide labels be placed on all new dishwashers. These labels are bright yellow with black lettering. When you're shopping for the best buy in a new appliance, using EnergyGuide labels can save you money. In addition to the EnergyGuide label, look for the ENERGY STAR label on dishwashers.
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