This seems like such an easy question, but I believe it is hard to come up with an accurate way to measure.
The easiest is to look at a time frame (winter) before putting in the furnace and then measure the energy usage for the next year and see what the difference is in usage.
But here is a problems with that method. The weather could be significantly different and the savings could be based on the weather change and not the windows. Another issue for me is that I want to make other energy efficient changes in my house, but if I do that now, I won't know which change caused the savings.
So my next thought was to measure a shorter period of time. Say overnight or on a daily basis. Again, if the temps are different it is not a valid measurement.
So, does anyone have a way of measuring before and after energy usage that takes temperatures into account to provide an accurate energy savings number?