The U-factor or Value measures the amount of heat that escapes through the product. The lower the rating, the better the window is at preventing heat loss (as imagined, this is more of a concern in northern climates). NFRC certified products require U-Factor ratings. In Florida, always look for the whole window U-Factor and select windows with a U-Factor at least as low as 0.75, but preferably lower than 0.65.

Additional Resources

UF IFAS Energy Efficient Homes: Windows and Skylights - http://edis.ifas.ufl.edu/FY1045 

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 http://www.dca.state.fl.us/fbc/publications/Fact_Sheets_0307/WindowSystems061506revised.pdf 

Efficient Windows Collaborative. September 2007. Fact Sheet: Selecting Energy Efficient Windows in Florida. http://www.efficientwindows.org/factsheets/florida.pdf