text: what are heating degree days and cooling degree days?   it's used for estimating fuel costs.  each town has an average number of heating degree days for the winter and average number of cooling degree days for the summer.  with those averages, you can calculate how long the air conditioner or heater will run for the year and then calculate the fuel costs.    you can calculate it from the average temperature each day.  example: average temperature for jan. 1 is 30°f. 65°f - 30°f = 35°f  jan 1 is one day, so 35° difference * 1 day = 35 heating degree days   average temperature for jan. 2 is 35°f. 65°f - 35°f = 30°f  jan 2 is one day, so 30° difference * 1 day = 30 heating degree days  you do this for every of the year and add it up.  jan 1 = 35 heating degree days jan 2 = 30 heating degree days jan 3 =  jan 4 =   add it all up and you get the number of heating degree days for the winter.