Is that really the bottleneck, though? They are already dramatically more efficient than combustive or resistive heat. But they are usually not drop-in replacements, and it's harder to retrofit older single family homes with them, vs a smaller apartment where you can just drop in a mini split and take out the old furnace.

Isn't that basically increasing the surface area? It makes sense at scale, but couldn't you reach same effect with 50% bigger area anyway? And I would expect that the surface area is already optimised to be close enough to what is needed. So for residential I wonder if there is any real gains there.