A heat pump is only moving heat from one place to another, it's not heating "resistively". So your energy cost is to run the heat pump, not to "heat" the house

A heat pump is only moving heat from one place to another, it's not heating "resistively". So your energy cost is to run the heat pump, not to "heat" the house. Having said that a typically, your system is designed for a 99% "heating" or "cooling" dry-bulb outside temp from the ASHRAE climatic design conditions based on your location. For example. Boston (Logan Airport) the 99% heating temp is 12.3F, so you'd size and design your system for this to get your heated space to 70F. There's a secondary heat source that fires if the temps drop below this (which happens, the 99.6% heating temp for Boston is 7.4F and it often goes below 0F). This is often resistive, but could be a hydronic coil if you had an on-demand gas combi-boiler for hot water. This is also used in an air-source heat pump for de-icing the condenser. A ground source heat pump (aka geothermal) doesn't have this problem, but requires a well to be drilled (100ft per ton of heating/cooling) to provide ground water as a heat source.