One measure used by energy companies to estimate customer consumption during the cold months is called "Heating Degree Days". The concept of "Heating Degree Days" is based on the assumption that people will start to heat their homes when the average daily temperature drops below 65^oF.

Now, "Heading Degree Days" is actually sort of a confusing term, because each day that the average temperature drops below 65 has its own number of "heating degree days". Here's what I mean.

Let's say that on a particular day, the high temperature is 48 and the low is 32. The average for the day is thus 48+32 divided by 2, or 40. Subtract this average from 65 to get the number of heating degree days for that day - in this case, 25. Energy companies do this calculation for each day on which the average temperature goes below 65, and then add all the numbers together, to get the total heating degree days for the year. That total helps them compare and allocate the energy needs of different locations.

In most cases, consumers will find a very good correlation between heating degree days and heating costs. Check this National Weather Service web page for daily and monthly calculations of heating degree days:

http://tgsv5.nws.noaa.gov/er/phi/clidat.htm