Re:"we can operate for weeks without taking in any energy at all"
I would say "go on and try to operate for a week without swallowing anything", but that would likely open me to a charge of attempted manslaughter.
Look it up or ask your doctor. You spend three days without even a drop of water and you are already on the verge of death. There are people who have gone on hunger strike for weeks (and I salute their resolve), but all of them drank water - sometimes sugared - in order to survive. And if you think that those people were in proper "operating" form after two weeks, well I think you need to go back to your doctor. Most of them were permanently confined to their beds after 15 days because too weak to get up.
We are not efficient energy users. We heat up as soon as we exercise, which means that we are wasting energy. If we were truly very energy-efficient, we would constantly be at room temperature. Take a look at humans in IR photos. You can clearly see the areas where skin is exposed - it's the whitest part of the picture.
We are not efficient in using energy, we are however quite efficient in recovering it from what we eat and in storing it.
