This paper will present the energy transfer variables of possibly the most efficient passive solar collector in use today - a skylight. A skylight makes use of the sun's light and its heat. A concept of U-value modifiers for skylights will be introduced. The concept of solar U-number modifiers (S-U-N modifiers if you will) is extremely simple. Essentially, this technique credits beneficial winter time solar gain against the more conventional U-value losses. Furthermore, lighting credit offsets conventional U-value losses and solar gain during the cooling season.

Except for the necessity of light control, we are discussing the energy savings potential of "unmanaged" skylights. Obviously, skylights with shades, blinds, apertures or those that are closed off in the evening are even better at conserving energy.