# Explanation of Degree Days

Degree Day measurement is a method of evaluating the outside temperature for use in calculating energy usage.

The National Weather Service records the high and low temperature for a given day.   The mean temperature is then subtracted from 65.  If the number is positive, the result is the number of heating degree days.  If the result is a negative number, the result is the number of cooling degree days.  An example would be:

`Today's High - 50  `
`Today's Low - 20`
`Mean Temperature - 35`
`Heating Degree Days = (65-35) = 30`

Heating oil companies keep track of heating degree days between oil tank fill-ups.   They then take the total number of degree heating degree days and divide it into the number of gallons that they filled the tank with.  This number is then referred to a s the "k factor".

If the heating oil company wants to fill your 250 gallon tank when it only has approximately 50 gallons remaining, they do the following calculations:

They multiply the "k factor" times 200 gallons.  This result is the number of heating degree days to wait  before filling the tank again.

This method is only an approximation, but short of a direct tank readout, is very reliable in about 90% of the homes served.  Factors such as wood stove usage, wind chill, temperature setpoint excursions, etc. will cause errors in the calculation.

R. L. Shields Associates has refined the calculation slightly by calculating the mean temperature based on 24 hourly observations rather only the high and low temperatures for the 24 hour period.

The National Weather Service defines the period, July 1 though June 30, as the heating degree day period.  The cooling degree day period runs from January 1st through December 31st.   