Great question, William.
We are all feeling the heat this summer both temperature wise and in our wallets, so it is most important to get a window that is going to keep the heat outside and keep you cool inside while saving you money on your utility bills.
Be sure you get a window that has Low E glass and Argon gas in order to ensure the maximum efficiency. The E- stands for emissivity. Really what it means is that a thin coating, usually silver or tin, is sprayed on the glass to reflect heat back to its source. This will keep the majority of that heat outside in the summer… which is where it belongs. I recommend the silver oxide coating. The tin is cheaper, but silver lasts. As my grandfather used to say “cheap ain’t good, and good ain’t cheap”.
In addition to the coating on the glass, a quality window will have an inert gas between the 2 panes of glass that is heavier than air, usually Argon or Krypton.
Here is a great way to understand how that works. If you took 2 glasses same size and filled 1 with water and 1 with dishwashing liquid and then you took 2 marbles the same size and dropped them into the glasses at the same time, which one do you think would reach the bottom the quickest?
Well if you guessed the one with the water you are correct because the dishwashing liquid is slowing that marble down. That is exactly what the Argon or Krypton gas does, it slows down the flow of your air-conditioned air thru that glass to the outside.
You need to ensure your window is Energy Star tested and rated. The Department of Energy through Energy Star has designed standard energy efficiency testing that will score a window and keep the results third party verified.
For a bit more information on “cool” window science and on the Energy Star rating system check out my article on Beat the Heat. And you might be interested in taking a look at the maps on The Lower My Energy Bill site. It will help you make the best window choice according to your region and state.
Hope this helps~