It depends on the installation, and this page will not invent a “typical share of the heating season” for backup strips. Electric resistance backup uses about 3,412 Btu of heat per kWh (a COP of 1). A cold-climate heat pump is supposed to do better than that at 5°F: ENERGY STAR requires a COP of at least 1.75 at 5°F and at least 70% of the 47°F heating capacity, verified with native controls. Those figures say the compressor can still produce heat in a cold snap. They do not say how many hours a given house will call the strips. A unit that meets design load at the local design temperature, with a well-set thermostat, may rarely use auxiliary heat. A unit that is undersized, that is allowed to call strips on a small setpoint bump, or that sees long stretches below its balance point may use them a lot.
What we will not do is turn a forum anecdote or an unsourced “only 5% of the season” claim into a table. If you need a number for a bill estimate, ask the designer for a bin-hour or hourly simulation that uses your load, the unit’s capacity curve, and the local weather — or look at a neighbor’s monitored system with the same equipment in a similar house, knowing that is still not a prediction.
Electrically, even occasional strip use can matter to the panel: a 10 kW kit is about 42 A at 240 V while it is on. See the backup-strips-and-panel page and the panel calculator. For operating cost, the calculator can illustrate heat at COP 1.75 as a cold-snap check, which is not an annual backup-heat total.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.