I'm not sure they're really concepts that can be compared directly the way you seem to be implying.
Usability is, based on Nielsen's definition, "is a quality attribute that assesses how easy user interfaces are to use", or as I would put it, the quality of how well the thing accomplishes the user's goals.
An affordance is what a design allows the operator to do. A button affords pushing, and sometimes it affords twisting, but it does not afford sliding. I think of affordances as the inverse of constraints, or the things that an design cannot do (which can be just as important).
A design that achieves good usability structures affordances (and constraints) in ways advantageous to the user's goal.