Usually heat pumps are installed with a second stage heating. In places with inexpensive gas, or low temperatures that will frequently be a gas furnace rather t

Usually heat pumps are installed with a second stage heating. In places with inexpensive gas, or low temperatures that will frequently be a gas furnace rather than resistive coils. Then the calculation is usually to take the electric price and convert it to price/BTU and divide by the heatpump COP at a given outside temperature to get a final system price/BTU and then set the compressor lockout second stage enable point where the gas price/BTU intersects the heatpump price/btu as it falls. Usually one should get this number before installing a new system for both gas and an electric second stage, and depending on system price/etc a decision can then be made about what to actually install. EX: in places with high electric prices and low NG prices or really cold temps, skipping the heatpump might be the right choice. OTOH, reddit/etc are full of people asking and finding out that their AC installer didn't do this calculation and just set the lockout temp at 40F or some other back of the hand value they have been using for a few decades. And yes, its possible to get heatpumps that work well below 35F, but they also have defrost cycles which absolutly destroy their efficiency because the defrost cycle is usually just running the heatpump in AC mode, while using the second stage to keep the air temp from falling in the house.