Pretty much all replacement windows are energy efficient now. They call them IGs which is short for Insulated Glass, basically a double pane window. They also now offer a Low E coating which is Applied to the glass before the 2 panes are sealed together, this helps reduced the amount of UV light entering your home, which also rdduces the fading of your hardwood floors and furniture. Whatever you decide just make sure you research the company. Find a company that has been around for awhile for warranty purposes and listen to what people are saying about them. Also check with your electric company and see what kinda discounts they might offer once you have your windows replaced. If Im not mistaken there is also a tax credit you can take advantage of.