Modified energy factor (MEF) is a number that describes how energy-efficient some appliances are; the higher the number, the more efficient the appliance. A more efficient appliance uses less electricity, which means it is better for the environment and will save you money on your utility bills.
MEF is a common specification used to describe washing machines. In order for a washer to be ENERGY STAR® compliant it must have an MEF of at least 2.0. The benefit of this rating system is that it gives you an easy way to compare different washer models to see which is more efficient. For a more accurate idea of how much energy each model will require over time, check out its kilowatt hours per year.