The sizes in universe between celestial bodies are really beyond the world on earth. We know the total mass of the sun is about 2×10^30 kg, which is almost 1/500 of the total mass of the sun! Of course this calculation method is really rough and even unscientific, but it reveals how less mass indeed supports the sun to be so powerful. The energy inside the sun is generated by nuclear reaction, so this formula is reasonable to apply. Assuming the sun can live for 10^10 years in total (10 billion years), the total amount of energy radiated by sun in this period of time is 1.2×10^44 J. A luminosity of a star is equal to the total amount of energy radiated per second. We firstly check how much energy is emitted in the whole life of the sun with luminosity. Indeed, the efficiency of converting matter into light and heat in sun is lower than we imagined, which can be figured out with a rough calculation. The sun is a spherical gas giving out light and heat, but it doesn’t mean that all parts of sun are turning the mass into energy.
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