"Excess usage" is the amount of electricity for the A/C that the contractor would use that is over and above the electricity that you would use for the same period of time if you were there. There is no way you could prove what that would be...and even if you could, as was pointed out above, we're talking about what amounts to pocket change.
Mountain, meet molehill.
No. Not if a unit is vacant. Then all usage is excess.