If you’re shopping for new windows, you probably already know that energy efficiency is one of the biggest selling points for replacement windows. That’s the way it should be. Efficient windows can regulate temperature and reduce the need for heating and cooling, rewarding you with lower utility bills. But before you’re ready to replace your windows, you’ve got a question: How do you know which windows are the most energy efficient?

First, you’ll want to look for windows with the blue ENERGY STAR seal. On average, ENERGY STAR-certified windows (and doors and skylights) can reduce your energy bills by 12%. If you replace old windows with new ENERGY STAR-certified models, you’ve already taken a huge step in the right direction. But there’s even more you can do to find the most efficient windows.

If you see an ENERGY STAR seal on a window, you’ll know that the window meets ENERGY STAR’s requirements for U-Factor and Solar Heat Gain Coefficient (SHGC) for your climate zone. Understanding what these terms mean can help you find even more efficient windows.

U-Factor refers to the degree to which a window allows non-solar heat transfer; examples include heat escaping through your window on a cold winter evening, or heat seeping in on a hot summer day. Energy-efficient windows reduce U-Factor by employing multiple panes of glass, sandwiching spaces filled with gases that don’t conduct heat very well, such as argon or krypton.

The less heat transfer, the lower the U-Factor. Energy-efficient windows should have U-Factors of .30 or less.

SHGC refers to the fraction of solar heat that passes through a window. The ideal SHGC varies, depending on the climate. Energy-efficient windows adjust SHGC by employing glazing, low-E coatings, and tints. In warmer climates, blocking the sun’s heat is preferable to letting it in, so the ideal SHGC values are lower. In cooler climates, collecting the sun’s heat is best, and ideal SHGC values are higher.

The Department of Energy has categorized the US into four different climate zone, ranging from the warmest zone, the Southern zone, to the coolest, the Northern zone. In the Southern zone, an ideal SHGC value is 0.25 or less; in the Northern zone, where it’s better to let in more heat, the ideal SHGC value can be 0.42 or even more.

Replacement windows from Renewal by Andersen are an excellent example of what can be done to increase energy efficiency. Renewal’s high-performance windows employ low-E coatings, which block out heat-producing infrared light while allowing natural, visible light to pass through. Their windows also employ multiple panes of glass; in between the panes is argon – a gas that conducts heat as two-thirds as well as the air we breathe.

It’s worth noting that there’s at least one other important factor in the energy-efficiency of replacement windows: Installation. In order to work as intended, windows need to be installed correctly and precisely. If any corners are cut – say the window doesn’t perfectly fit the opening, for example – then performance can be compromised. If you’re set on having the most energy-efficient windows you can, be sure that the quality of your installer is every bit as good as the quality of the window itself.