What people actually pay to heat with a heat pump
21 households who published their own winter numbers, normalised so they can be compared. A factor of 12 between best and worst, in cold climates with almost identical degree days.
Field notes
The reasoning behind the model, written out where it can be checked.
I asked three contractors the same question and got three different answers
Two gave a confident balance point without knowing anything about the house. The third refused and listed what he would need. Then I paid for the load calculation and found both confident answers were about twenty degrees off.
What two federal field studies found when they measured real heat pumps
PNNL measured a median COP of 1.9 at 0 to 5F across 22 instrumented homes. NREL found resistance strips outconsuming the compressor at 2 of 12 sites. Both are right, and the difference is the finding.
Most heat pump calculators use the wrong gas price
The error is easy to make, hard to see, and it moves the answer in most of the country.
The 3.5 to 1 rule describes two specific machines
Derive the threshold rather than quoting it and you find it moves by more than 60% across ordinary equipment.
Backup heat is where cold climate bills come from
A system that delivers 15% of its heat from resistance strips loses a quarter of its efficiency advantage.