Remember that Luminosity is the total amount of energy produced in a star and radiated into space in the form of E-M radiation. The Sun radiates ergs/sec.
Start with the Sun
It's easy. Measure its (apparent) brightness, measure it's distance and use the inverse square law.
The apparent brightness is a measure in photon energy passing through a square whatever at the distance of the Earth. One way to determine the total energy release in photons is to multiply the photon energy per square whatever, times the total area of the sphere with radius 1 AU (remember an astronomical unit is the distance between the Earth and Sun) in units of square whatevers.
(Note that a normal-sized human emits 100 watts in IR power).
Strangely enough a black horse in a field on a sunny day absorbs around 800 watts = 1 horsepower.
ergs/cm /sec (roof area) gives the energy beating down per second in Solar photons.
Roof Area: 100m 100m m cm .
Is this alot? Total campus usage is 3.5 MW.
We have the energy per square cm per second:
The Surface Area of a sphere with radius R= 1AU is:
Still the same, 1 solar luminosity. The Martian ``solar constant'' will be smaller by a factor of 1.5 x 1.5, but added up over the entire surface of the sphere centered on the Sun with radius 1.5AU, the luminosity is the same. Luminosity is an intrinsic property of the Sun.