If you have a minute, here’s a great place to learn what heating and cooling degree days are: Degree Days – An Introduction
From the website:
Heating degree days are a measure of how much (in degrees), and for how long (in days), the outside air temperature was below a certain level. They are commonly used in calculations relating to the energy consumption required to heat buildings.
It’s basically a way to quantify how hot or cold a place is by looking at discrete points in time instead of using an average. As a simple example, let’s say it’s 80°F (daytime) for half the day, and 50°F for half the day (nighttime). The average temperature for this day is 65°F which might imply that you don’t need to turn the air on.
Realistically, though, you had to turn the air on during the day when it was 80 out. What HDD and CDD do is break down the day into smaller chunks and provide you a number that you can use to determine how much you might have needed to run the heating or air conditioning that day.
As a practical example, you can actually look up average HDD and CDD per year for your region. I run my stage 1 heat pump for 7 minutes per degree day. My region of Indiana has 5699 HDD, which means my stage 1 heat pump will run for 664 hours in an average year. You can also do the same math by day or whatever other time period.
Finally, it makes little sense to have a metric for “runtime”. A house in Indiana and a house in Texas have drastically different summer temperatures. A metric of “runtime per degree days” normalizes both homes so the effect of the outdoor temperature (mostly) goes away.
Make sense? I probably need to write a guide on this. It’s really powerful data but it took me a while to understand. 