I want to correct some comments posted here. A modern heat pump (made within the last 10 years) will produce more heat than the electric power it uses into the low teens or single digits. The issue with heat pumps is that the colder it gets, the less heat it will produce (colder outside air contains less heat), but the heat it is making is still more efficient than electric strips. So at 20 deg it may only produce 50 or 60 percent of the nameplate rating. At the same time, the colder it is, the home's heating requirements go up. The temperature where the heat produced and the heat required are equal is called the balance point. Below that temp, some electric aux is needed, and above none is needed. The balance point of a unit depends on efficiency, sizing, heat loss, etc but is generally in the upper teens to low 20's for a properly sized unit.
Defrosting can bring on the aux heat when above the balance point. About 90% of all units sold use what is known as Time-Temp Defrost. This can control can result in unneeded defrosts, especially at lower temps. A unit with Demand Defrost will only defrost when needed, based on temperature sensors. Demand defrost can save 10-15% on your bill, depending on where you live, and other factors. There is a defrost controller on the market, the Emerson 47D01U-843 which is an upgrade/replacement controller for most units on the market. You could get one installed for $300-$500 by your HVAC guy. That can eliminate unnecessary defrost cycles.
The other factor is the user. DO NOT set back the temp unless you will be gone for days. Constant temp setting is the best option. After a setback, the requirement for a rapid temp rise will cause most units to kick-in aux heat. Some modern thermostats that are programmed for a setback will slowly rise temperature and not trip the aux heat, but in general the savings from a setback are exaggerated.
Last item is your unit's condition. If you have a refrigerant leak, or an undercharge, efficiency will be reduced and the outside unit will tend to ice up more frequently and therefore defrost more often. Unfortunately properly checking the charge level in cold weather is difficult, and not done properly by many service techs.
paul