The solar radiation outside the earth's atmosphere is calculated using the radiant power density (Hsun) at the sun's surface (5.961 x 107 W/m2), the radius of the sun (Rsun), and the distance between the earth and the sun. The calculated solar irradiance at the Earth's atmosphere is about 1.36 kW/m2. The geometrical constants used in the calculation of the solar irradiance incident on the Earth are shown in the figure below.
The actual power density varies slightly since the Earth-Sun distance changes as the Earth moves in its elliptical orbit around the sun, and because the sun's emitted power is not constant. The power variation due to the elliptical orbit is about 3.4%, with the largest solar irradiance in January and the smallest solar irradiance in July. An equation  which describes the variation through out the year just outside the earth's atmosphere is:
H is the radiant power density outside the Earth's atmosphere (in W/m2);
Hconstant is the value of the solar constant, 1.353 kW/m2; and
n is the day of the year.
These variations are typically small and for photovoltaic applications the solar irradiance can be considered constant. The value of the solar constant and its spectrum have been defined as a standard value called air mass zero (AM0) and takes a value of 1.353 kW/m2. The spectral irradiance is given in the Appendix.