At what outside temperature does a heat pump lose it’s heating efficiency? i.e. before the aux./emergency heat kicks in?
Well in the old days it was around 25 I am not sure if it has improved much.
I also believe it to be in the 25-30 range.
It has to do with the balance point of the building not the equipment.
The point at which a heat pump can no longer absorb heat from the exterior air has to do with the refrigerant characteristics and system design. This temperature is well below the point at which the thermostat calls for a second stage heating.
Depending on how efficient the house is, is the point at which second stage heating energizes. If your house leaks 24,000 BTU/hr and your heat pump cannot produce that amount, the temperature will fall inside the house and the second stage heater will energize.
My house has a 17 F balance point.
Those 25-30F ranges may be a common balance point for the construction standard for your area but can not be considered for all areas and equipment.
Heat loss rate is proportionate with the indoor and outdoor temperature differences. So obviously it would change from thermostat settings made from occupant to occupant.
Thanks David 
Thanks to all.
I’m actually in the Orlando area so the need for heat is limited (fortunately) but has been necessary (!!) for the last 2 weeks. Unit is an Amana (Goodman), 3 ton, 16 SEER, R-410 refrig. installed approx.2 months ago.
Here is a link on heat pump balance point.
http://www.arthurhewett.com/id26.htm#hp_balance_chart
The balance point where the heat pump’s balance point is determined the structure’s heat loss vs the heat pumps output capacity.
With newer higher efficiency and two stage units. The units first stage would be sized for the cooling load. Second stage being a much higher btu rating would be able to go to a much lower outdoor temperature. Example a structure sized for a 2 ton btu cooling load would have a 4 ton two stage condensing unit. The balance point for the 4 ton 2 stage unit may have a balance point of 10 degrees outdoor temp. A single stage 2 ton unit would have a higher outdoor temp balance point.
Gary, great info, thanks.