Measuring the luminosity of the sun is equivalent to measuring the rate at which the nuclear reactions produce energy in the core of the sun. But we think we know exactly how those reactions work:
4 H atoms -> 1 He atom + energy
where the energy is released because some of the mass of the Hydrogen atoms is converted to energy.
Mass of 1 Hydrogen atom: 1.673 x 10^-24 grams
Mass of 1 Helium atom: 6.644 x 10^-24 grams
This nuclear reaction has an input (4 H atoms) and two outputs : 1 He atom and energy.
Since the He atom has less mass than 4 H atoms, that difference in mass must have been converted to energy. If the energy is produced via E=mc^2, how much energy is produced from doing this reaction on a group of 4 Hydrogen atoms? (Hint: How much mass is converted into energy?)