Efficiency and standards in United States

Top-loading and front-loading clothes washers are covered by a single Federal Standard regulating energy consumption. The old Federal Standard applicable until January 1, 2011 included no restriction on water consumption; washer manufacturers faced no legal restriction on how much unheated rinse water could be used. Energy consumption for clothes washers is quantified using the energy factor.
But after new mandatory Federal Standards were introduced, many US washers were manufactured to be more energy- and water-efficient than required by the federal standard, or even certified by the more stringent Energy Star standard. Manufacturers were found to be motivated to exceed mandatory standards by a program of direct-to-manufacturer tax credits.
In North America, the Energy Star program compares and lists energy efficient clothes washers. Certified Energy Star units can be compared by their Modified Energy Factor (MEF) and Water Factor (WF) coefficients.
The MEF tells us how many cubic feet of clothes are washed per kWh (kilowatt hour) and is closely related to the configuration of the washer (top-loading, front-loading), its spin speed and the temperatures and the amount of water used in the rinse and wash cycles.
Energy Star residential clothes washers have a MEF of at least 2.0 (the higher the better), but the best machines may reach 3.5. Energy Star washers have also a WF of less than 6.0 (the lower the better).