To the right you can see a window efficiency rating sticker from the National Fenestration Rating Council (NFRC). It's a sticker you should see on every new window, though sometimes, the actual ratings will vary a bit. Let's look at the ratings provided, and what they mean:
U-FACTOR (U.S./I-P): Lower = Better
Possibly the most common rating factor is U-Factor (sometimes called "U-Value"). It measures how well the window prevents heat from escaping, and lower numbers are better. The lower the U-value, the better a window resists heat flow, so the better its insulating value. U-Factor ratings generally fall between 0.20 and 1.20. Most windows considered "energy efficient" will have a U-Factor of 0.30 or lower.
SOLAR HEAT GAIN COEFFICIENT: Lower = Better
Another fairly common rating factor is Solar Heat Gain Coefficient (SHGC). It measures how well a product blocks heat caused by sunlight. As with U-Factor, lower numbers are better. SHGC is always a number between 0 and 1, and the lower the number the less solar heat it allows into the house.
VISIBLE TRANSMITTANCE: Higher = Better
Visible Transmittance (VT) measures how much visible light comes through a window, and it's expressed as a number between 0 and 1. Unlike U-Factor and SHGC, higher numbers are better when it comes to VT, because it means the window is clearer. Because U-Factor and SHGC are improved by adding physical coatings to the glass in windows, improving those numbers generally decreases VT. As such, window efficiency is a sort of balancing game between improving energy efficiency while keeping windows as clear as possible.
AIR LEAKAGE: Lower = Better
Air Leakage (AL) indicates how much air can pass through a window, and as you'd expect, less (i.e. lower number) is better. The actual AL rating indicates the cubic feet of air passing through a square foot of window area, which leaks through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.
CONDENSATION RESISTANCE
Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100.
