What is Energy Factor?

Legal Definition
An energy factor is a metric used in the United States to compare the energy conversion efficiency of residential appliances and equipment. The energy factor is currently used for rating the efficiency of water heaters, dishwashers, clothes washers, and clothes dryers. The term is used by the United States Department of Energy to develop and enforce minimum energy conservation standards under the Energy Conservation Program.

The higher the energy factor, the more efficient the appliance should be.

Although the term energy factor is used to compare the relative efficiency of these appliances, the metric is defined differently for all four appliance categories. The energy factor is expressed in terms of site energy, which excludes losses through energy conversion. All of these efficiency metrics are defined by Department of Energy test procedures.
-- Wikipedia
Legal Definition
Overall energy efficiency of an appliance or equipment. Contrast to relative efficiency. Calculated as total energy consumed in active, cycling, and standby modes. Different types of devices have EF computed differently. EF typically displayed very visibly on an item. For example, for a water heater, EF is based on three factors: (1) how efficiently the heating element transfers energy from the outlet to the water, (2) what percentage of energy is lost from cooling of unused, stored hot water, and (3) how much energy cycling between active mode and standby mode uses.