Blowing up a planet is no small feat, but the Death Star's laser manages it easily. But the energy consumed is a bit much. Like a full week's worth of the sun's total output.
Yes, in order to blow up a planet the size of earth, you'll need to harness about 2.2 x 10^32 joules. That's a ridiculously high amount of energy. And that's not even enough to really put on a show; if you want to do more than just dissociate the planet into a diffuse cloud, you'll need way more.
I don't know about you guys, but the lack of realism in the Star Wars films has really started pissing me off. I'm going to write a strongly-worded letter to George Lucas. [Science Blogs]