Modified Energy Factor (MEF) is an equation that takes into account the amount of dryer energy used to remove the remaining moisture content in washed items, in addition to the machine energy and water heating energy of the washer. MEF is the energy performance metric for clothes washers qualifiying for the Energy Star rating, a joint program of The U.S. Department of Energy and the U.S. Environmental Protection Agency. The higher the MEF, the more efficient the clothes washer is.

MEF is the quotient of the capacity of the clothes container, C, divided by the total clothes washer energy consumption per cycle, with such energy consumption expressed as the sum of the machine electrical energy consumption, M, the hot water energy consumption, E, and the energy required for removal of the remaining moisture in the wash load, D.

The equation is:

MEF = C / (M + E + D)

Water Factor is the number of gallons per cycle per cubic foot that the clothes washer uses. The lower the water factor, the more efficient the washer is. If a clothes washer uses 30 gallons per cycle and has a tub volume of 3.0 cubic feet, then the water factor is 10.0.

WF is the quotient of the total weighted per-cycle water consumption, Q, divided by the capacity of the clothes washer, C. If a clothes washer uses 30 gallons per cycle and has a tub volume of 3.0 cubic feet, then the water factor is 10.0.

The equation is:

WF = Q/C

Currently, all Energy Star qualified clothes washers must have a minimum MEF of 1.72 and a water factor of 8.0.