An efficient window is one that will keep you comfortable year-round and help reduce your energy usage in your home. Typically, windows are National Fenestration Rating Council (NFRC) tested for winter-time heat loss, or u-value, and summer-time heat load, or Solar Heat Gain Coefficient (SHGC).
They are both expressed as a number between 1 and zero, with the lower the number the better. For a frame of reference, aluminum single pane windows usually have a u-value of around a .79 rating, and an SHGC of .89, which means they would let out a lot of heat in the winter and let in too much heat in the summer.
A typical double pane would be a .49 for u-value and .79 for solar heat gain. Newer, Energy Star Rated windows need to be (at minimum) .30 for u-value and .25 for SHGC.
Our Anlin Catalina Windows with Infinit-e plus high-performance glass standard EXCEED the ratings by as much as 45% and have u-values and SHGC well below what a lot of other manufacturers offer. Often window companies barely make the Energy Star rating which means in just a few short years they will not qualify!
Energy prices will not be getting any cheaper, so do not settle for windows that are minimal compliant for they will cost you more in the long run.