electric heat pumps (with appropriate backup heat),
Why do you need a backup for heat pumps?
Efficiency drops off at lower temperatures.
Efficiency does drop, but it is not the reason you need backup heat. Most air source heat pumps are still more than "100%" efficient compared to resistive heat down to about 0 F outdoor temperature. Backup heat is needed because the heat capacity goes down as outdoor temperature goes down. You may get 36,000 BTU/hr at 60 F, but at 20 F it is only about 20,000 BTU/hr (varies with design of unit). Unfortunately, the amount of heat you need goes up as outdoor temperature goes down.
When sizing heat pump systems, the capacity vs outdoor temperature is plotted on the same graph as the heat load. They form a nice "X" shape and point they cross is called the "balance point". That is the outdoor temperature you need auxiliary heat to keep the building warm.