Here's the really good link for figuring out how much money your windows are costing you: http://homeenergy.org/archive/hem.dis.anl.gov/eehem/95/950708.html
The story behind this:
We had a visit from a windows salseman. Now, I'd been thinking about replacing my windows with new ones since the windows had their seals broken and looked really bad. One point the man mentioned was how great these windows were at stopping heat loss. He had a demonstration involving a heat lamp and how you could feel the heat through a regular window, but not one of the really cool windows. Then he quoted us a price around $6,000 which was a lot more than I was willing to pay.
He also said that maybe the windows could knock $45 off our heating bill, cutting it in half. Now, I admit that would be nice. But I was skeptical. We already had double payned windows. So I used the info in the link above to calculate my savings if I replaced the six windows in the house that really needed it. This is just to give me an estimate if new windows would be worth it.
Here's my set up:
6 windows, 3 that are 3x4 and 3 that are 4X4 for a total of 84 square feet
we keep our house at 68 degrees F, so it is about 30 degrees F difference between outside and in
Gas costs about 1.096 per therm (as per our house bill)
I assumed my furnace had an efficency of 80%, since that is what it is supposed to have.
Assuming we have just plain double glazed windows, we are loosing $15 a month through those 6 windows.
With new and improved double glazed windows with energy coating and argon gas the energy loss would be $10.70 a month. A savings of $4.40, or about 29%.
Umm... $4 a month is not anywhere close to $45 dollars a month. If we had single glazed windows, the savings would be much larger. This just agrees with what I've read elsewhere, new energy efficient windows will not pay for themselves any time soon. (Hmm... it would take 125 years for these windows to pay for themselves.) You upgrade windows because you like the way new windows look, or your old windows are ugly, or you want your house to heat up less from the afternoon sun. Now one thing this doesn't take into account is that new windows might be better caulked and fit better, so there would be less heat loss around the window, so maybe there would be more savings that way.