When I was a kid in the 80s my house had an electric heat pump and the aux (resistive) heater coil didn't kick on until about 0F (-18C). You could tell because

When I was a kid in the 80s my house had an electric heat pump and the aux (resistive) heater coil didn't kick on until about 0F (-18C). You could tell because it smelled a bit when it happened, which was pretty rare. I guess it probably kicked in some more times overnight and I didn't notice, but overall the heat pump did the lion's share of the work. Heat pumps have become much more efficient since then. I replaced an old and rusty unit on my first townhouse and cut the electric bill by $100-$200/month in the middle of summer and dead of winter.