For St. Louis, HVAC systems are designed for 5000 Heating Degree Days, but you can find the actual data for each year here: http://www.crh.noaa.gov/lsx/?n=cli_archive
What the hell are heating degree days:
To calculate the heating degree days for a particular day, find the day's average temperature by adding the day's high and low temperatures and dividing by two. If the number is above 65, there are no heating degree days that day. If the number is less than 65, subtract it from 65 to find the number of heating degree days.
For example, if the day's high temperature is 60 and the low is 40, the average temperature is 50 degrees. 65 minus 50 is 15 heating degree days.
We use this data to level our calculations and comparisons about equipment and efficiency from year to year.
You can get your utility data on line and with it you can separate the data into base load and seasonal loads. This post explains how: http://riles-files.blogspot.com/2012/08/residential-energy-base-load.html
At the end of it you'll have an idea of how a home of your size, in an area similar to yours, compares to others in energy consumption. Chances are the lower the number the greater the comfort and, certainly, the lower the bills.