You'd think, wouldn't you, that it would be much more energy efficient to use a single, reusable, rewashable, ceramic cup - rather than dozens of disposable styrofoam versions. But it's not as easy as that. What about dishwashing? Yes, there's been a study:

The results are extremely sensitive to the amount of energy the dishwasher requires for cleaning each cup. Hocking's choice for the dishwasher, requiring 0.18 MJ/cup-wash, is barely less than the manufacturing energy of the foam cup, 0.19 MJ/cup. If Hocking had chosen even a slightly less energy-efficient dishwasher as his standard, then the reusable cups would never have broken even with the foam cup.