We replaced all the windows in our house w/ new double pane vinyl windows last summer. I envisioned the gas bill getting cut in half. That hasn't happened. The bill shows your usage and the average daily temperature for the current month and that month in the previous year. I've saved absolutely nothing. My electric bill, however, has been about a third of what it was last year. I hoped that my windows were just confused and cut into the wrong bill, but the reality is that almost all of the light bulbs in our house are now CFL bulbs. So in review, thousands of dollars in new windows saved me no money, but $4 light bulbs have already saved me $100. I heartily recommend the light bulbs.