How much difference energy efficient windows make, depends on how high their energy rating is. They can help you save on your heating bills, how much depends on other factors like the size and type of home you have. They can help you maintain the warmth of your home in winter months, and how much heat your windows let in during summer months. They can also help you use less energy, reducing your carbon footprint. Replacing your old double glazing with double glazing with an A+ energy rating could help you reduce your carbon usage by 140kg per year*.
*Source Glass and Glazing Federation. † A typical detached house has 23.7m2 of windows. Based on replacing uPVC double glazed windows with a 12-14mm sealed unit and gas central heating with A+-Rated thermally efficient windows.