If you are willing to reverse it the amount of energy can be 0 or a close approximation.

See: http://en.wikipedia.org/wiki/Reversible_computing

Note that this is the computing equivalent of a straight line.

Would that be related to the reversible computing I heard about a few years ago, in terms of reserving the energy?

Recovering would be a better term I think. You could read the wikipedia article to see if it is what you remembered.