Calculate the approximate Main Sequence lifetime of the Sun using the following assumptions to find out how much energy the Sun has available and how long that will power the Sun: When H fuses into He, 0.7% of its mass becomes energy. 1/10 of a solar mass of H gets hot enough to undergo fusion. The Sun loses energy at a rate of 1 solar luminosity, or 4e33 (4x1033) erg of energy per second