AC is extremely efficient for what it's doing, but it only works because the temperature difference is relatively minor. Where they can get away with it, they

AC is extremely efficient for what it's doing, but it only works because the temperature difference is relatively minor. Where they can get away with it, they use something similar to AC for heating in the US. But it doesn't work everywhere for heating. Consider that even in the hottest climates in the US, you're cooling your air by 30 degrees. But in the coolest climates, you're heating your air by 60 degrees. My sister has this system in Philadelphia, but when it gets cold enough the more inefficient raw electric heating kicks in, and that really chews through electricity like no other.