# Shading Design - Surface Outside Face Sunlit Fraction

I am trying to use this E+ variable to gauge effectiveness of a few shading strategies in conjunction with some more complicated daylighting tools. The sunlit fraction definition says it looks at unreflected beam solar radiation at each time step to calculate the fraction. Anyone knows how it actually determines the unreflected versus reflected solar radiation? Is there a daylight (lux) or energy (W/ft2) threshold by which E+ determines that? I wasn’t able to find anything in the E+ documentation!

It is important to know how this unreflected beam radiation is determined since we are trying to model a detailed shading strategy in LightStanza to calculate a similar metric (Sunlit Fraction) for the whole year then use simple shading objects (overhangs) in E+ to mimic that with performance by comparing to a metric that looks at the number of hours direct sun hits the windows. That threshold definition dictates how we would calculate from LightStanza.

Surface Outside Face Sunlit Fraction []

The fraction of the outside area of an exterior surface that is illuminated by () beam solar radiation. Equals Surface Outside Face Sunlit Area divided by total surface area.unreflected