I think this will act as the earlier build of the Battery Cover that they launched with SP1. It is both, actually. The tablet mode has a battery and it is extended (if you will) when docked on the keyboard. The larger battery in the keyboard is where you get the longer life. My guess is with lower level usage the thing can get 1/2 the rated life without the keyboard base. Of course this is 100% variable as it always depends on what you're doing during that period of time.