In colder climates we welcome the suns heat and light most of the time. Once we capture the heat, we don’t want to give it up. Less than half the suns energy is visible. Longer wavelengths are infrared, which is felt as heat. Shorter wavelengths are ultraviolet.
Low- E coatings work best in heating climates when applied to the internal, or interpane, surface of the interior pane. Low-E coating reflect up to 90% of the long wave heat energy, while passing shorter wave visible light. In heating climates, they reflect long wave radiation back into the house while passing visible light. Visible light is absorbed by floors, walls and furniture and is reradiates from them as long wave heat energy, which the low E coating keeps inside.
Solar-heat -gain-coefficient (SHGC) is the fraction of available solar heat that successfully passes through a window. SHGC is based on percentage of available rather than on a percentage of what comes through a window. It considers various sun angles and the shading from the frame.
Look for U-values of the whole unit of 0.33 or better. Look for warm edge spacers to help reduce condensations around colder areas of the frames.
Visible Transmittance (VT) is a measure of the admittance to natural light. High VT can save energy because you need less artificial light. VT for clear glass is around 90% and as low as 15% for tinted glass. The most common VT value is about 60%, which looks clear. Any value below 50% begins to look dark or reflective.