This is how I replied to the very same review comment about heat pump
operating temps, and elect strip heat.

This screen capture shows the efficiencies of the PTHPs used for the
baseline. The Unloading curves show that the system is designed to
calculate efficiencies and capacities as outdoor temperatures vary.

This screen capture shows that the PTHPs will operate down to 17 degrees
F, and the electric strip heat is the second stage of heating if the
compressor cannot keep up.

The secondary capacity curve prevents the strip heat from being
activated when the outdoor temp is above 40 F.