The efficiency of a window, door or skylight can't be considered in abstract. Weather conditions should be taken into account. Different climates demand windows with different charactheristics, namely expressed in terms of unwanted heat losses (U-Factor) and unwanted heat gains (SHGC)... 

Climate & Window Glasses

In cold climates it's particularly important to use low-E coating glasses to reduce heat losses through the windows.

In hot climates the most important is to use Low E2 glasses to limit solar heat gains (low-E2 glasses are particular efficient in reducing the infrared and some ultraviolet rays, besides being good insulators and transmitters of visible light).

See also: Window Glasses and Panes

Better yet, it's convenient to consider different types of windows to mixed climate conditions, that is, to different climate zones.

USA Four climate zones for Windows

In USA, Energy Star considers four climate zones for qualified windows, doors, and skylights:

Northern: colder climates, demanding mostly heating windows
North/Central: temperate mixed climates
South/Central: temperate mixed climates
Southern: warmer climates, demanding mostly cooling windows

Source: www.energystar.gov

See also: Detailed State US table with climate zones for windows, doors and skylights

The U-factor values and the SHGC in those four American climate zones are the following, for windows:

Northern climate: U-factor: <0,35; SHGC: any
North-Central climate: U-factor: <0,40; SHGC: <0,55
South-Central climate: U-factor: <0,40; SHGC: <0,40
Northern climate: U-factor: <0,65; SHGC: <0,40

Energy Star Climate Zones for Skylights

Northern climate: U-factor: <0,60; SHGC: any
North-Central climate: U-factor: <0,60; SHGC: <0,40
South-Central climate: U-factor: <0,60; SHGC: <0,40
Northern climate: U-factor: <0,75; SHGC: <0,40

 

AUSTRALIA
Climate Zones for Windows

Image Source: www.greenhouse.gov.au

Australian Window Energy Rating Scheme (WERS) considers three large Australian climate zones. Windows are rated from 0 to 5 stars for both cooling (summer) and heating (winter).

 

 

CANADA
Climate Zones

Canadian authorities also consider four different types of climates and qualified windows, doors and skylights.

 

 

 

US climate Zones
for Windows, Doors and Skylights

TYPE OF WINDOWS, DOORS AND SKYLIGHTS
State	Climate Zone Window
Alabama	South/Central or Southern
Alaska	Northern
Arizona	Northern or South/Central
Arkansas	South/Central
California	South/Central
Colorado	Northern or North/Central
Connecticut	Northern
District of Columbia	North/Central
Delaware	North/Central
Florida	Southern
Georgia	South/Central or Southern
Hawaii	Southern
Iowa	Northern
Idaho	Northern
Illinois	Northern or North/Central
Indiana	Northern or North/Central
Kansas	Northern or North/Central
Louisiana	Southern or South/Central
Kentucky	North/Central
Maryland	Northern or North/Central
Massachusetts	Northern
Maine	Northern
Michigan	Northern
Minnesota	Northern
Mississipi	South/Central or Southern
Missouri	Northern or North/Central
Montana	Northern
Nevada	Northern or South/Central
New Jersey	Northern or North/Central
New Mexico	North/Central or Northern
North Carolina	North/Central or South/Central
North Dakota	Northern
Nebraska	Northern
New Hampshire	Northern
New York	Northern
Ohio	Northern or North Central
Oklahoma	South/Central
Oregon	Northern
Pennsylvania	Northern
Rhode Island	Northern
South Carolina	South/Central
South Dakota	Northern
Tennessee	North/Central or South/Central
Texas	South Central or Southern
Utah	Northern or South/Central
Vermont	Northern
Virginia	North/Central
Washington	Northern
West Virginia	Northern or North/Central
Wisconsin	Northern
Wyoming	Northern