Yes, the reports out on air source heat pumps in places like England are that they don't work in extreme cold, requiring a backup (usually heating oil there). All of this gets very expensive. I suppose in Coastal CA they'd be fine, along with a well insulated house. My brother (a mechanical engineer) installed a water source heat pump using his well water (a much more stable 60 degrees or so) many years ago. I'd suppose this would work much better than air source (if available to the owner). If homeowners adopted water recovery cisterns (increasingly attractive in water parched or fire threatened areas), this could also serve as the water source for heat pumps. Currently the irony is abandoning one's natural gas heating for electric air source requires electricity coming largely from...natural gas! We need to carbon capture this electricity source and/or perhaps convert to hydrogen/green gas delivery in place of natural gas delivery, plus wait for renewables to mature quickly. Also, what ever happened to solar hot water? Those things were REALLY efficient (about 5 year paybacks, much better than photovoltaics). One problem with a bunch of non grid systems was that the homeowner almost had to be an engineer to keep up with it all!