The solar constant is the amount of solar radiation incidence that is received at a distance of 1 astronomical unit (AU) from the Sun. It is presently calculated as being 1,366 W/m2.

Due to the fact that the earth rotates around the sun and it's distance depends on the time of year, the actual direct solar incidence that Earth receives varies from 1,412 W/m2 in January to 1,321 W/m2 in July.

If you want to calculate specific solar incidences for a certain distance in AU's, use the following formula:

Calculating Solar Incidences for the Solar System:

E = C • (1/Distance)²

Where

E: Incidence in W/m2.
C: Solar Constant at 1 AU (1,366 W/m2)

Below is a table of solar incidences for mission planning, whether for spacecraft thermal control to spacecraft power generation.