The "E" in low-E stands for emissivity, and a clear coating of metallic oxide on these windows keeps the heat inside the house in the winter and outside in the summer.
Typically, this coating is used on external storm windows in houses that don't have double-pane windows. The technology comes in soft coatings and hard coatings. The soft coatings go between layers of glass, while the hard coatings go on the outside.
Low-E windows typically cost between $60 and $110 each. That's 10 to 15 percent more than clear glass storm windows, but they definitely have benefits: They can reduce heat flow through the glass by half, and that will help reduce heating costs by 10 to 20 percent [source: NAHB Research Center].