It depends on the nameplates, and this site will not invent a “typical 3-ton house” standby size. A whole-house automatic system has to cover whatever may come on when the transfer switch closes, including the heat-pump compressor’s start and, in cold weather, electric resistance backup if that heat is still enabled. Honda’s home table does not list central heat pumps, so there is no Honda default to lean on. The portable-generator-and-heat-pump page is the same warning at a smaller scale: starting current and backup strips are the usual surprises.
What to collect for the dealer or electrician:
- Outdoor-unit running current (RLA or FLA) and locked-rotor or listed starting current, at the voltage on the nameplate.
- Backup or auxiliary heat kilowatts, and whether the control can lock that heat out during generator operation.
- The rest of the automatic load list: well or sump, refrigeration, air handlers, kitchen loads if they will stay on.
Enter the motor and resistance loads you already know as custom 240 V items in the calculator to see how fast the running total climbs, then let the standby installer apply their starting-kW method and the ATS scheme. Dual-fuel (gas furnace backup) can shrink the generator because a furnace blower is a much smaller load than a heat pump plus strips. That 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.