The Daylight factor is the reference parameter used in Energy Standard EN15193:2008 to assess daylight contribution in the energy performance of buildings. But its efficacy in putting in relation buildings energy performance, daylight availability and visual tasks is now a subject of discussion in literature. In fact, the daylight factor is a static indicator independent from building geometry and environmental parameters. From the energy point of view, it has still to be assessed if it can ensure when the switch of artificial light is needed, representing therefore the most disadvantageous outdoor illuminance conditions. The goal of this study is demonstrating if the daylight factor calculated in standard conditions (Overcast sky) is conservative respect to that calculated using the diffuse component of the horizontal illuminance under CIE (Commission Internationale de l'Éclairage) Clear sky. If so, Daylight factor (D) might be applied, possibly with suitable corrections, to estimate the energy requirements for lighting, keeping the results on the safe side. 