Don't they just start working essentially as effective resistive heater below some temperature ? So theoretically just oversizing it a bit should've been enough for the once in 10 years cold snap event.
Woodstove with some supplies probably cheaper than oversizing tho
So many people will say that but even at -13F the LG Red heat pump I have is rated for a COP of ~2. Which means it is still twice as efficient as a resistive space heater. Now capacity does fall off and that can cause issues since heating demands rise as temperature drops but that is an initial design constraint that just has to be designed around.
NEEP has a great website to find all kind of specifications for heat pumps. Below is the link to the unit we have at our house in Southern New Hampshire.
https://ashp.neep.org/#!/product/53914/7/25000///0
I've heard that with heat pumps you want to be very careful about getting the sizing correct. So if your plan is to get a heat pump and improve home insulation you need to do the insulation part first so you don't end up with an oversized heat pump.
This is, basically, my plan out here in the midwest. I want a wood burning stove insert into my fireplace, improved insulation, and finally a heat pump. Long term I'll completely eliminate natural gas.
Woodstove with some supplies probably cheaper than oversizing tho