I've kind of thought that the energy to create the planet-destroying weapon wasn't generated by those ships. Instead, I'm thinking that it's merely being channeled from some other universe/subspace continuum. I mean, they have the technology to open up wormhole-like passages from their space to ours, it doesn't seem all that unlikely that they could do the same thing with energy.

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"You're not quite evil enough. You're semi-evil. You're quasi-evil. You're the margarine of evil. You're the Diet Coke of evil, just one calorie, not evil enough."
-The good Doctor and his son, Scot