If you have records of Direct & Diffuse radiation:
The Solar Model requires values for the direct transmissivity, and for the diffuse proportion.
Integrate your data to give monthly totals of Direct and Diffuse radiation.
Calculate the equivalent values using HemiView – Direct radiation integrals are given in the Total row at the bottom of the DirAb sheet, Diffuse radiation integrals are calculated by multiplying the DifAb value in the values sheet by the appropriate month value in the DifMonth sheet.
By trial and error, adjust the values of transmittivity and diffuse proportion to give best fit between your measured irradiance at the point, and the computed irradiance using HemiView. You may find that a single solar model setting will not give a good fit for the whole year, so it may be appropriate to find several sets of solar model settings that work at different times of the year, and only use the corresponding outputs for those times.
If you have records of Sunshine duration:
Calculate the percentage sunshine as the number of actual sunshine hours divided by the total possible sunshine hours (sunrise to sunset). Multiply this percentage by 0.9 to give an approximate value for the direct transmissivity.
Set the diffuse proportion to a value of 0.1 for direct transmissivity >0.8 or < 0.2; to 0.2 for direct transmissivity between 0.2 – 0.4 or 0.6 – 0.8; to 0.3 for direct transmissivity between 0.4 – 0.6.
IMPORTANT NOTE
If you have distinct seasons with very different radiation characteristics, you will need to do these calculations for each season, so you have different solar model settings for different parts of the year. Only use your results for the period when the solar model is valid.