Well,
As I know both 8 cores don't operate at one time. 4 cores are clocked at higher freq and the other 4 cores are clocked low freq. At low freq it consumes about 1/10 power than it consumes at high freq. So at this point apps can't use 8 cores at one time. Also as I know there's a hidden 9th core -can't be used by apps or android- which decides which core groups is suitable at that point. So portable devices like this can have more battery life than the ones that have only 4 cores running at high freq.