[abhibeckert]

The efficiency mostly comes from _moving_ heat that already exists, instead of creating new heat.

Depending on the specifics, that's often about 5x more efficient.

Counterintuitively - the "heat" source doesn't have to actually be hot. In theory as long as it's above absolute zero it can be used as a heat source to heat up the inside of your home. It's the same for cooling - even if it's hot indoors and really hot outdoors, you can still use a heat pump to make it even colder inside the hosue.