This is serious overanalysis, (Just shut up and watch the damn movie, Ed!!) but this website has interesting things to say about shooting beams at planets.
http://www.theforce.net/swtc/ds/#superlaser
"The most basic consideration pointing towards some kind of superphysics is the magnitude of the device's energy output. The minimum necessary magitude of the blast that shattered Alderaan is roughly ten thousand times what would be released if the entire mass of the battle station's machinery was annihilated with an equivalent amount of antimatter."