Backup heat strips are often the difference between “the heat pump will fit” and “you need a heavy-up.” They are electric-resistance elements, so their nameplate is essentially the kW rating. A 10 kW strip is about 42 A at 240 V; a 15 kW strip is about 63 A, before any other load. That can exceed the remaining room on a 100-amp service once a charger or an electric range is also in the picture.
The 100-amp EV-plus-heat-pump page walks through this with labeled examples: about 79.3 A with a 6 kVA heating value, 96.0 A at 10 kVA, and 116.8 A at 15 kVA, which exceeds 100 A in that walk-through. Those kVA figures are examples, not a typical strip package — use the nameplate on the air handler or heater kit.
Ask the HVAC quote to state the backup-heat kW in writing, and put it in the panel calculator. Dual-fuel (gas furnace backup) or a cold-climate unit that can meet design load with little resistance heat is an electrical strategy as well as a comfort strategy.
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.