Replacing windows on an existing home isn't always a good idea because it can be quite expensive and only offers an energy bill savings of 7 to 15 percent [source: ENERGY STAR]. But if you're building a house, choosing energy-efficient windows during construction provides enough energy savings to cover the added cost per window (usually $15 more than a generic window) [source: Sachs]. If you decide to find a greener window option for your home, there are a few things to keep in mind. Most importantly, you need to understand thermal transmission (or U-factor, as it is commonly known) and solar heat gain ratings. These ratings measure the amount of heat that is lost in the winter months and gained in the summer months, respectively. The lower the rating for each, the more energy efficient your window will be. If these ratings are making your head spin, don't worry. ENERGY STAR has a rating system for windows as well. Generally speaking, if you look for the ENERGY STAR label, you'll find a good, eco-friendly product. They even break windows down by climate zones to help you find the perfect window for your area. Remember that ENERGY STAR products should be at the same price point as other standard windows, or at least make up for the extra upfront money in energy savings in a relatively short period of time.