So, lower numbers mean less heat loss. Most window manufacturers now give the U-value for the entire window unit, including the frame. Good windows are U-0.40. The best source of information is The Certified Products Directory published by the National Fenestration Rating Council. SHGC expresses the amount of solar heat that penetrates the window compared to the amount that strikes the outside.
Lower numbers mean less heat entering the building.
SHGC will eventually replace SC in manufacturer's literature. The NFRC Directory should start listing SHGC starting in January 1995. This affects daylighting and the ability to see objects and views outside. The human eye can adjust to wide variations in visible light, so it compensates to some degree for low VLT numbers. Clear glass has a value of 90 percent.