Post offices are the tip of the iceberg. Every office building is like that. Think about all the 30+ story sky scrapers in every major city. They all run heating and AC throughout the night, when no one except maybe a few guards and maintenance people are in them. We are spectacularly wasteful, which is why the average American consumes something like 200 times the energy of the average person in the third world. BTW, how many miles per gallon does that car that you drove to that post office get?