These days, energy efficiency is one of the best amenities a replacement window can offer; it’s also quickly becoming one of the most inevitable features as more manufacturers opt for Energy Star-rated products to meet consumer demand for lower energy costs and improved interior climate management. But how exactly is a window’s energy efficiency assessed, and how much value can it truly yield for you in the long run?

Concerning the value question, the answer turns out to be: quite a bit—according to the U.S. Department of Energy. The department estimates that Energy Star-rated products can save you 7-15% on your annual household energy bill based on your location and the type of window you’re replacing. Traditionally, single and double pane windows are standard issue for many residences, but these models don’t offer an ideal insulation factor, allowing for unwanted air seepage in and out of your home. Recent, energy-efficient innovations, though, have drastically altered traditional window construction, and gone one step further by employing the application of dense gases, like argon or krypton, to further improve unit insulation.

Let’s get specific by taking a closer look at the Energy Star rating itself. It’s sounds official, but what’s the criteria behind it? What exactly are you getting when you purchase replacement windows that meet this standard?

The answer is similar: quite a bit.

The Energy Star program was developed by the U.S. Environmental Protection Agency with the intent to create a set of standards for assessing window energy efficiency. From a construction standpoint, Energy Star-rated windows feature spacer systems that maintain an appropriate distance between the glass panes to improve insulation and reduce condensation within the unit. They also offer Low-E, or low emissivity, glass coating engineered to deflect ultraviolent rays from penetrating your glass and fading nearby furniture.

The Energy Star standard is not a simple eye test, though; there are specific metrics that are also measured, like a window’s U-Factor, which measures the rate of heat loss. There’s also the Solar Heat Gain Coefficient (SHGC), which measures how much solar radiation a window absorbs and transfers into your home. It’s important to note that SHGC is a layered metric assessed in two ways: whole window and center of glass (COG). The former evaluates the absorption and transmittance rate for the whole unit while the latter is more of an assessment of the window’s glaze.