We haven't replaced our windows yet, but I thought I had read somwhere that "bad" windows account for 20% of energy loss in a home. "Good" windows could reduce that loss by 50% which would equate to an energy reduction of 10%. Here's a far more official website:

https://www.energystar.gov/index.cfm?c=windows_doors.pr_savemoney

Here is a link that might be useful: How Much Money/Energy Will You Save

Totally depends on the house, windows selected, windows being replaced, and a whole slew of other factors.

Most of our customers do report back instantaneously on how much more stable the temps are as well as how quiet the home is.

Great advice from WoW. Will new windows save energy? YES. How much? Impossible to say given all of the variables, but 10-15% is not out of the realm of possibility. RUN from any companies that are still around claiming numbers like 40 or 50 % energy savings.
You could also look at an energy audit to determine the best ROI on energy saving improvements in your home. Just be weary of the goofballs offering "12 point energy assessments" trying to pass that off as a real energy audit.

You definitely will notice a savings on your bills. The BIGGEST savings will be the hard to quantify " comfort factor". You will be MUCH more comfortable in both the winter and summer. Thats the first thing my customers mention, how big of a difference in terms of comfort they experience. This is true with a solid window.

Big yes on comfort factor, actual $$ savings hard to say because we also updated our electronics and whatever savings we earned on more efficient windows, was guzzled by electronics, which are great electricity users. Save here, lose there, we're lucky to break even

It's interesting to note that the best savings (from the Energy star table) are in the Northeast, even better than Alaska! I guess that's more because of high energy prices in the NE and low ones in Alaska!

That and they build crap-ola (I believe that is the technical term) up here.

Areas that are really THAT cold seem to get the worst in materials and construction quality.

Not sure I understand. The NE has some of the best built houses in the country. I know MA had the strictest building code in the country at one point. Once you get out of New England the quality drops off dramatically.

KSC1,

My point is that in areas of the country where the climate is not as severe as Alaska, many builders over the last 20 years have built sloppily and chosen questionable materials.

I am not blaming the builders singularly here either. Codes have allowed for the use of materials and techniques that I disagree with.

The lack of exterior sheathing is one of them that I think is really poorly thought out.

Window quality is always questionable in most track built homes.

The codes have come quite far but there are also some huge issues with the exterior insulation schedules that they are adopting and mark my words...there are going to be some huge issues with interior condensation if they don't modify the codes.

Compare these construction evolutions to how people in severe climates build and their ways are much more long term proven.

I think it is safe to say that we built "smarter" 50 years ago.

Yes...materials and technology have come quite far since then and we are using less energy but homes 50 years ago were designed to take advantage of passive heating, ventilation, and shading.

They also weren't built with a ton of glass...although I can't fault them for that. That part helps us a good bit :-)