Why only 0.7% efficiency?

In hydrogen-to-helium fusion, 99.3% of the original mass remains as helium. Only the mass deficit (binding energy difference) is converted to energy.

How much hydrogen does the Sun burn per second?

About 620 million tonnes per second, of which about 4.3 million tonnes become energy.

Fission converts about 0.09% of mass to energy; fusion converts about 0.7%, making fusion roughly 8 times more energy-dense per unit mass.

Calculate the energy released by converting a given mass of hydrogen to helium via stellar fusion (0.7% mass-energy conversion): E = 0.007 m c^2.

Hydrogen Mass

Energy Released

629,128,625,115,772.400 J

Energy (megatons TNT)0.15 Mt

Energy Released vs Hydrogen Mass

In the pp-chain and CNO cycle, four hydrogen nuclei fuse into one helium nucleus. The mass difference (0.7%) is converted to energy via E = mc².

E = 0.007 × m × c²

where m is the hydrogen mass consumed and c = 299 792 458 m/s.

The Sun converts about 600 million tonnes of hydrogen per second, releasing 3.8 × 10²⁶ W.

Energy from fusing 1 kg of hydrogen.