Degree days are used to estimate energy requirements for the heating and cooling of buildings. They are also used to compare one period to another, usually monthly or annually.
To calculate a heat degree day, the high and low for the day are averaged and then subtracted from 65. For instance if today's high is 40 degrees Fahrenheit and the overnight low is 20, then today would have 35 heat degree days. Monthly data can be compared to prior years to determine if a month was colder or warmer and thereby influencing the amount of energy that was required to heat the building for that particular month.
If you used more energy this month than you normally do, compare the heat degree days for the month versus the same month's in prior years. If it has been considerably colder, this could be the reason for increased usage. If however, the degree days are similar or less, you may have an issue with your heating system.
If you have a heat pump, you may want to verify that the heat pump is working properly. As a heat pump is the only heating system with essentially two different heating systems, (the heat pump portion and the auxiliary heat system) the heat pump system could fail and yet you would very well never realize it until you receive your monthly bill. As the auxiliary system is a much lower efficient heat source, considerable more energy would be required to maintain the temperature setting in your home.
Cooling degree days are used to estimate and compare air conditioning needs. Instead of subtracting from 65, the daily temperature average is added to 65.