Adjective : 2000 square feet of usable office space. ,That saw is no longer usable.    From Dictionary.com. 

The sitting-room of the little home which she enjoyed for the last two or three years of her life, was a reflex of her character in modesty, simplicity, and usableness.    From Wordnik.com.  [The Angel Adjutant of "Twice Born Men"] Reference

But it further became evident that in such a series there inevitably occurs a waste in the usableness of energy.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy] Reference