My friend is debating with me and he says "A 500 megaton nuke has 20% of the Suns power. Is this true?

He might have meant the power of the nuclear reaction. The power of the Tsar Bomba reaction (50 megatrons) was a few percent of the power output of the Sun (~10^26 W). If the 500 Megaton nuke released all its energy in a few nanoseconds, it could have 20% of the Sun's power.