I have 4 space heaters for emergencies. I gave 3 of them (two oil filled plug in radiators and one 240v 30a “garage heater”) to a friend that has a house with 100% heat pump heat. Heat pumps work great, above 15F. Below that they can’t pull any heat from the outside air.
He survived last night, came home this afternoon, house was 65F, cats were happy, fitchen faucet was dripping but appeared “slushy” so he is on edge. Older house, kitchen sink is on an exterior wall, not much he can do but let it drip.
I kept ONE oil filled radiator cause I needed a backup. It’s in the barn, set to 50F, barn minisplits are useless below 15f also, but that one heater is keeping the barn at 42F. Why do I care? Well the barn is a workspace for me (not today!) and I have some plants overwintering in there that shouldn’t freeze.