Choosing energy-efficient windows is probably the most complex aspect of building. In this article I'll offer some advice to simplify the seemingly innumerable choices. The focus is on northern climates, where heating energy costs significantly exceed cooling energy costs.
Most windows now include a specifications label. All these labels include a U-factor which indicates how much heat is lost out the window, and a solar gain (SHGC) which indicates how much heat from the sun is gained through the windows. A good window should have a U-factor less than 0.3 and SHGC more than 0.5. It should use an Inex spacer or Super Spacer at least 5/8" wide. A window with a cheap aluminum spacer will lead to much more condensation on the window in winter.
Use fixed(picture) windows where possible instead of operating (slider/casement). Not only are fixed windows cheaper, they have smaller frames than operating windows which allows for more glass area and a higher SHGC rating.
According to LBNL Resfen, a window rated U 0.29 and SHGC 0.56 facing south in Portland, ME will have a net gain 76,430 BTU of energy per square foot of area. This is the amount of heat energy from 22 kWh of electricity or about 2/3 of a gallon of heating oil. The same window facing north will have a net loss of 17,500 BTU of energy per square foot of area, so minimizing north-facing windows in a new build reduces heating energy use.