What g is
The share of solar heat that passes through the glass into the room. Ug tells you how much heat escapes in winter; g tells you how much gets in during summer.
Why standard triple glazing does not solve summer
IG Low-E 4+16+4+16+Low-E 4 [44 mm] has a Ug of 0.6 and keeps the heat in well in winter, but its g-value is 53%: in summer it lets in more than half of the solar heat.
What does solve it
A solar control coating. IG Sun4+16+4+16+Low-E 4 [44 mm] has a Ug of 0.5 and a g-value of 32%, while still letting through 64% of the light. It is better in both winter and summer.
What that means in watts
When 1000 W of sunlight falls on the glass, standard triple glazing lets through 527 W, and triple glazing with a Sun coating 324 W. In summer, the air conditioning has to remove that difference.
Sources: Ug to EN 673, g and LT to EN 410; values from Guardian’s performance calculator.