Let's think here, Master Roshi's Kamehameha at full force destroyed the moon when Goku turned into a Giant Ape, Master Roshi's power level is 139. So, to be fair we'll say a power level of 300 is required to blow up the Earth. Now, considering Dragon Ball GT, SSJ4 Goku's power level is considered to be 1,000,000,000 after absorbing the Dragon Balls. Now, let's think, at the power level of 300 we can destroy a planet, at a power level of 1 billion: we can destroy what?
That's all I have to say.
-Bread