We almost never charge our EV to 100%, to not degrade the battery faster.

But, if you are going to offer “absorbing energy from the grid as a service” the capacity you have to offer can only be

  <absolute max that you are willing to charge your battery to> - <minimum that you are willing have your battery sitting at> 

There definitely could be some gap there, but it does depend on the car sitting at less than “full” (however you define full).