Heating and cooling engineers have found a way to relate the typical climate conditions of different areas to the amount of energy needed to heat and cool a building. The term they use is "Degree-Days" using a base temperature of 65F (18C). A heating degree-day is counted for each degree below 65F (18C) reached by the average (between the highest and lowest) daily outside temperatures in the winter. For example, if on a given winter day the high is 40F (4C) and the low is 20F (-1C). This is 35F (19C difference) below the base temperature of 65F (18C). So, on that day, you would have gone through 35 (19) heating degree-days.
a type of degree day used for estimating fuel consumption for warming the indoor environment to a base temperature, generally to 65 degrees Fahrenheit; one heating degree-Day is given for each degree that the day's average temperature is below the base temperature