I'll just add a little more explanation here since our site goes pretty in depth and I imagine most here would only be interested in a brief overview:
So buildings have what is known as a heating base temperature, which is the outside temperature above which heating is not needed inside the building. This is not the thermostat temperature inside the building (say 20 C), it is actually lower because of various factors like people and electrical equipment generating "free heat" inside the building, and how well the building is insulated to retain that free heat.
The base temperature varies from building to building, but let's say, for example, a fairly-well-insulated home might have a heating base temperature of 14 C. If the temperature outside is 14 C or above, that building will stay perfectly warm inside on its own, without the heating system needing to come on.
But, if the outside temperature drops below 14 C the building will need some heating to keep it comfortable inside. How much heating it needs will depend on how much the temperature drops below 14 C, and for how long.
And this is what heating degree days quantify. Here is an example diagram that demonstrates quite nicely how they are calculated (using a base temperature of 14 C):
The neat thing is that the heating degree days for any period of time represent all the relevant temperature variations across that period of time, and assuming you used an appropriate base temperature, are proportional to the heating energy consumption over that period of time. So for example you can have just one figure that represents the heating degree days across an entire week/month/year, and that will encapsulate all the relevant temperature variations across that week/month/year.
If January had 200 heating degree days, and February had 300, you can expect the heating energy consumption of the building to be 50% greater in February than in January. (Assuming you have chosen the right base temperature for your building that is!)
Compare this with knowing that the average temperature across a week/month/year was 12 C. What does that tell you about how much heating was needed in that week/month/year? Not a lot, cos you have no idea how much it varied within that time. This is the case even within a single day, since the temperature can vary a lot within a day.
Hence why people in the energy-saving business would typically use degree days rather than temperature data :)
(That said, hourly temperature data or similar is good for more sophisticated building simulations. But those are a lot more involved. On the simpler end of the spectrum degree days are a much better choice.)
So buildings have what is known as a heating base temperature, which is the outside temperature above which heating is not needed inside the building. This is not the thermostat temperature inside the building (say 20 C), it is actually lower because of various factors like people and electrical equipment generating "free heat" inside the building, and how well the building is insulated to retain that free heat.
The base temperature varies from building to building, but let's say, for example, a fairly-well-insulated home might have a heating base temperature of 14 C. If the temperature outside is 14 C or above, that building will stay perfectly warm inside on its own, without the heating system needing to come on.
But, if the outside temperature drops below 14 C the building will need some heating to keep it comfortable inside. How much heating it needs will depend on how much the temperature drops below 14 C, and for how long.
And this is what heating degree days quantify. Here is an example diagram that demonstrates quite nicely how they are calculated (using a base temperature of 14 C):
https://www.degreedays.net/images/heating-degree-days-calcul...
The neat thing is that the heating degree days for any period of time represent all the relevant temperature variations across that period of time, and assuming you used an appropriate base temperature, are proportional to the heating energy consumption over that period of time. So for example you can have just one figure that represents the heating degree days across an entire week/month/year, and that will encapsulate all the relevant temperature variations across that week/month/year.
If January had 200 heating degree days, and February had 300, you can expect the heating energy consumption of the building to be 50% greater in February than in January. (Assuming you have chosen the right base temperature for your building that is!)
Compare this with knowing that the average temperature across a week/month/year was 12 C. What does that tell you about how much heating was needed in that week/month/year? Not a lot, cos you have no idea how much it varied within that time. This is the case even within a single day, since the temperature can vary a lot within a day.
Hence why people in the energy-saving business would typically use degree days rather than temperature data :)
(That said, hourly temperature data or similar is good for more sophisticated building simulations. But those are a lot more involved. On the simpler end of the spectrum degree days are a much better choice.)