When a pokemon card says 20x the number of energy do you do 20 damage if there is no energy on a pokemon or would it be zero? Mewtwo for example, is it 20 plus the number of energies?
Let's say you talk about this Mewtwo : http://pokeplayer.com/cardex/next_destinies/98-mewtwo-ex
If Mewtwo has 2 energies and the Defending Pokémon has 0 energies, that would do 20 × 2 = 40 damage.
If for a weird reason you were able to use X Ball without having any energy attached, and 0 energies on the Defending Pokémon, that would do 20 × 0 = 0 damage.