The solar luminosity (L_☉), is a unit of radiant flux (power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun.

One nominal solar luminosity is defined by the International Astronomical Union to be 3.828×10²⁶ W. This does not include the solar neutrino luminosity, which would add 0.023 L_☉, or 8.8×10²⁴ W, i.e. a total of 3.916×10²⁶ W (the mean energy of the solar photons is 26 MeV and that of the solar neutrinos 0.59 MeV, i.e. 2.27%; the Sun emits 9.2×10³⁷ photons and as many neutrinos each second, of which 6.5×10¹⁴ per m² reach the Earth each second). The Sun is a weakly variable star, and its actual luminosity therefore fluctuates. The major fluctuation is the eleven-year solar cycle (sunspot cycle) that causes a quasi-periodic variation of about ±0.1%. Other variations over the last 200–300 years are thought to be much smaller than this.