In the case of windows and glazing, because they admit the sun's energy, the U value concept can be taken a step further. Even in the winter the sun can shine and help to warm a room. Over a typical heating period the amount of useful heat from the sun entering through windows can be subtracted from the normal U value (a measure of the heat leaving through the windows) to give an overall "effective U value". This is dependent upon the direction the window faces. In the case of south facing windows with Pilkington K Glass double glazing units, the normal U value of 1.9 W /mK is reduced to an Effective U value of virtually zero. In other words, the amount of heat lost is balanced by the heat gained from the sun.
Effective U value can simply be regarded as "heat out minus useful heat in".
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