Heat transfer is expressed as U-value. The lower the U-value, the more energy efficient the window.
Solar gain is an asset in cold climates but a liability in hot ones. Solar gain in measured on a scale of 0 to 1, which is called the solar heat gain coefficient (SHGC). A higher number indicates more solar gain. Suggested ratings are as follows: Northern U.S. – .55 or above; Southern U.S. – .40 or below; temperate climates – .40 to .55.
Air leakage is measured in cubic feet per minute (cfm)per square foot of glass. The higher the number, the leakier the window. A rate of less than .30 indicates a good seal.
Visible transmittance (VT) is the amount of light that comes through a window. This is one factor you can judge yourself simply by comparing the views through different windows.