(Direct Solar Radiation x percentage of window in direct sun) + Diffuse Solar Radiation (ASHRAE Fundamentals 1993 27.35)

There are two components of solar radiation - the direct component and the diffuse component. The direct component is solar radiation traveling from the sun, through the atmosphere and directly onto the building. It helps to think of the direct component as the part that causes shadows. The diffuse component is slightly more complicated. Diffuse solar radiation is broken up into two categories - sky and ground reflected. The sky component takes into account the brightness of the sky, and is simply the solar radiation that comes from it. The ground component comes from direct solar radiation that is reflected off the ground near the building. Diffuse solar radiation is non-directional and cannot be blocked by shading devices. It helps to think of the diffuse component as north light or daylight on a very cloudy day. It does not cast shadows.

The north value in the SHGF chart is a good proxy for the diffuse component. Subtracting the SHGF value for the selected orientation by the north value yields the direct component. The percentage of window in direct sun comes from the Solar 2 simulation. These calculations are easiest to do in a spreadsheet.