The National Fenestration Rating Council (NFRC) provides energy performance labels (See illustration) on windows, doors, and skylights voluntarily tested by a third party for how well the product will perform the functions of cooling your home in the summer, heating your home in the winter, keeping out wind, and resisting condensation. The label also lists the manufacturer, describes the product, provides a source for more information, and includes a rating for how much light comes through the product. Basically, the NFRC label allows you to comparison shop. Keep in mind that the label is voluntary so not all products will have it.

Additional Resource:
EnergyStar®  

UF IFAS Energy Efficient Homes: Windows and Skylights

American Architectural Manufacturers Association

Building a Safer Florida, Inc., University of Florida's Program for Resource Efficient Communities, and the Florida Department of Community Affairs. May 2006. Window Systems

Efficient Windows Collaborative. September 2007. Fact Sheet: Selecting Energy Efficient Windows in Florida. 

Window Selection Tool

National Fenestration Rating Council