The article doesn’t go into it, but a key advantage they have is that heat pumps move heat, rather then trying to generate it. So they can move a lot more heat into your house than would be generated by running the electricity they use through a resistor. This makes them effectively more than 100% efficient (the exact amount depends on temperature) as compared with burning a fuel or resistive heat.
To determine heating degree days for your area, you set a baseline temperature (18C is kind of standard in Canada) take the average temperature on each day, and sum the difference between that and the baseline temperature for every day of the year (zero if temp is above baseline). So if the average temp one day was -10C, it would be a 28 heating degree day.
It allows approximation of building heating demand. Some standards (Passive House) use heating degree hours for finer detail, which makes sense because there can be fairly significant day/night temperature swings.
Here’s a site where you can calculate what your location is. And here’s what Wikipedia says.
Huh, TIL…