Here is a great Technology Connections video which lays out why the sizing issue might not be as bad as you're thinking. The gist is that many people are steered away from heat pumps (or sold very expensive oversized units) by outdated industry thinking which vastly overestimates load calculations.
I won't try to make his case here, but he is pretty convincing that a reasonably sized heat pump system can be sufficient even in very cold climates (by US standards).
It's all theoretical. In real life, retrofitting heat pumps into exisinting homes is a bit different, as those homes are neither made for air heat sources (at least in my region), nor they are insullated well. You might say, dough, insulate it, but it is easier said than done, as a lot of those old homes relied on their leakage for ventilation, not to mention that is very expensive investment (much more expensive than the heat pump itself).
I would reccommend keeping backup heat source when installing heat pumps into older homes. A simple wood stove, used during cold spikes, could be enough.
My country has subsidies for heat pump installations into older homes and it had requirement to get rid of chimneys to get that. After the whole europe energy crisis thing, this requirement is gone now. Turns out having alternatives is a good thing :)
This is not theoretical: my heat pump purchased in 2021 worked well for four straight days of below 0F. Kept our temp at 68 inside. We live in an old home from 1917 with poor insulation between the brick exterior walls and drywall.
This is why I opposed historical designations. Most old buildings are obsolete and need to be destroyed. There are only a tiny number of buildings that are really historic, sure save them, but the vast majority are just old and need to be knocked down for something better. If it isn't taught in history class then it isn't historic.
It is easy to see the loss of something old. However the opportunity cost from not building new is something that is very hard to get people to understand.
It will still work, just not as efficently as in normal temperatures. So you're using 50kWh for the 3 days a year it's that cold and 10kWh for the other 362 - better than using 20kW every day.
This is actually terrible and supports their point. Heat pumps are not sized as if they were running at a mere 100% or less, they are sized for the btus they normally produce, and aim to run close to 100% of the time, meaning they will be too small and not be able to do the job when they drop below 200-300% efficiency, unless they are grossly oversized. That leads to short cycles which makes condensation and moisture in the summer and wear and tear all year and less electrical efficiency because starting a pump and blower costs more than running it.
Current inverter types that can run slow do allow them to be oversized but it's still not great. Those are more expensive and have more failure points in electronics too.
