This is not the whole story because not every heating day is equally cold. I have a high end cold climate heat pump in Colorado (which works great btw). I use about 1/3 of my total annual heating energy in January, despite heating for >6 months of the year. I'll use 10% of my annual energy budget for a long weekend if its -10F, and that's all heat pump (I don't even have backup strip heat). It would be 20% if i was using electric resistnace for those 4 days. Electric resistance is really not great, so folks really should get the best heat pumps they can that cover the coldest normal days. It's fine to install strips as a true backup but you're going to have some very high bills and high carbon if you're using it 20-30 days/year. If its hydro/nuclear power you'll still come ahead on carbon but that's not the case everywhere.