What is the Sun's total luminosity in watts?

The Sun’s total luminosity is 3.828 × 10^26 watts.

Luminosity is the total power a star emits in all directions, unlike irradiance, which measures power received over a particular area. The Sun’s value is the nominal solar luminosity, written L☉, adopted by the International Astronomical Union in 2015. In older references it may be rounded to 3.8 × 10^26 W or 4 × 10^26 W, but 3.828 × 10^26 W is the precise conventional value used in modern astronomy.

This enormous output comes from nuclear fusion in the Sun’s core. Hydrogen nuclei combine through the proton–proton chain to form helium, releasing energy that eventually works its way outward and escapes as electromagnetic radiation and other particles.

A frequent error is confusing luminosity with the solar constant. Luminosity is the Sun’s entire power output; the solar constant is the much smaller power-per-area received near Earth, about 1,361 W/m². The watt is joule per second, so the luminosity describes energy emitted every second.