The solar spectral irradiance calculated from the Solar Spectral Irradiance (SSI) Climate Data Record (CDR)
which measures the solar energy input at the top of the Earth's atmosphere at a given wavelength (SSI band) for  quiet, low, moderate and high solar activity levels and the Maunder Minimum. 
Detail under doi:10.7289/V51J97P6. 
The irradiance is given in W m-2 Hz-1 and is computed using a Sun radius of 6.96342e8m and a distance of 1.495978707e11m a Earth radius of 6378e3m and a TOA height of 1e5.