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This question was submitted on behalf of a user:

I'm building a code in Java to calculate the global solar radiation that is incident on an area that is randomly oriented on Italian territory and I would like to use the data in this page

  1. I have noticed that Global Horizontal Radiation data do not match the sum of Diffuse Radiation data and Direct Normal Radiation, why?
  2. In order to achieve my objective, what data should I use, Global Horizontal Radiation data or the sum of Diffuse Radiation data and Direct Normal Radiation data?
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  1. Answer from subject expert:

    Answer 1:

    You are correct because the “Global Horizontal Radiation” is relative to a horizontal plane, tangential to the surface of the Earth.  The diffuse is also computed relative to the same horizontal plane.  However, the direct normal is computed relative to a plane perpendicular to the direction of the sun.  Thus,

          GHI = Diffuse + Direct-horizontal

    An estimate of the Direct-Normal is obtained using the Cosine of the Solar Zenith angle

         Direct-horizontal = cosine(solar zenith angle) x Direct-Normal


         Direct-Normal = Direct-horizontal / cos(solar zenith angle)

    The relationships between these flux components are explained more thoroughly in SSE Methodology Section 6 which we encourage the user to review carefully to better understand how the parameters are derived for this version. 

    Answer 2:

    Please review the information on tilted surfaces also found on the SSE Methodology document. 

    The PDF for this document can be found at


    Please be advised that SSE is about to be replaced and the new data is available at  We  welcome you to respond with additional questions.