Calculating Degree Days

Heating degree day measurements have been around for a long time. Simply put, it is a way to measure how far the (cold) weather travels in a day, thus you are able to track and accumulate daily, weekly, monthly, seasonal and annual degree days. Calculating degree days allows for seasonal or annual comparisons, which can also be used in the budgeting process.

While some on-line services have begun to provide more sophisticated degree days, by factoring in wind chill, humidity and weighting certain times of the day, to provide a more enhanced value (another topic), the basis for computing degree days hasn’t changed.

Simply put degree days are the number of degrees the average temperatures were below 65 degrees, for a 24-hour (1 day) period.

Why 65 degrees? In the early days, someone decided when it got below 65 degrees they needed to back up to a heater and get warm, thus the threshold was born. The correlation between the high/low temperature and the Average Temperature.

That being said, let’s see how to calculate the degree days for a given day. Let’s assume the high for the day was 70 degrees and the low was 40. Add the two together, divide by two, to calculate the average. 110/2 = 55. Thus, the heating degree days for the day are 10. If the K-factor was 5, they consumed two gallons of fuel, for heat, that day.

But what is a K-factor? It is a burn rate. How many degree days must elapse for the structure to burn one gallon? This is easily determined by tracking degree days systemically and knowing the current reading each time a delivery is made.

If you make a delivery in November, and the YTD degree days are 1500, and you make the next delivery in December, and the YTD degree days are 2000, and the tank consumes 100 gallons, the Burn Rate/K-Factor (500/100) is 5. So, in this example it takes 5 degree days to burn a gallon of fuel.

You can easily think of degree days as miles and K-Factor as miles per gallon. Same principle. It takes your car 5 miles to burn a gallon of fuel.