What are Heating Degree Days (HDDs)?
Heating degree days are indicators of household energy consumption for space heating. It was found that for an average outdoor temperature of 65 degree Fahrenheit, most buildings require heat to maintain an internal 70 degree temperature. The higher number of HDDs indicates more energy is required for space heating.
How heating and cooling degree days are computed?
Average daily high and low temperature is greater than 65 degrees F = cooling degree day
Average daily high and low temperature is less than 65 degrees F = heating degree day
How do we use these degree days?
The most common use of degree days is for tracking energy use. Without degree days, comparing the energy used over two periods would be analogous to calculating the miles per gallon rating for your car without knowing how far you had driven. If you wanted to know if the attic insulation you added over the summer was saving energy, you would use your energy bills to determine how much "fuel" was used before and after the retrofit. Then, using the degree days, you could determine "how far you went" during those periods. Instead of calculating miles per gallon, you would determine kWh's per degree day or therms of natural gas per degree day.
When comparing energy use, you should also consider other energy uses that are not impacted by weather such as lights, appliances, etc. You can estimate the energy used for these purposes by examining the energy used during typical temperate months such as May and October when little heating or cooling energy is used. The energy used during these periods reflect your base monthly consumption. Subtracting the base use from the total consumption during a winter month will yield an estimate of the energy used just for heating. It is also important to consider the usage period reflected in your energy bill since days in a month vary. You can allow for this by comparing over a longer period, such as an entire heating season or several months.