Dual fuel is also useful if electricity is expensive but gas is cheap and if you know what the efficiency curves look like for your appliances. I recently did

Dual fuel is also useful if electricity is expensive but gas is cheap and if you know what the efficiency curves look like for your appliances. I recently did this calculation and effectively when ever a heat pump would be less than a COP of 4, my gas furnace would likely cost me less money for the same amount of heat output. Our electric rates have effectively doubled in the past 5 years while gas has dropped in price. Obviously this situation is going to be region-specific and gets even more complicated with time-of-use billing. Ideally a smart thermostat would be able to consider all these variables and pick the most cost effective heat source for a dual-fuel setup but I don't think any of them are quite that useful yet. Only a handful of centrally ducted heat pumps on the NEEP list (https://ashp.neep.org/#!/product_list/ https://ashp.neep.org/#!/product_list/) have COP >4 at 5F outdoor temperatures.