Hibernate creates an image on thhe HDD and the whole system goes dormant like a bear in a snow cave. That does use less power than component sleep. All are choices in Power Management. What you don't want to do is have it keep running with the lid closed - that can cause overheating in some laptops be cause part of incoming cool air is through the keyboard.
In sleep mode my laptops use about 2 watts. At home I run them on the adapter with the battery removed. Leaving the battery attached would be a very bad idea since Li-ion rechargeables degenerate at a far faster rate when they are topped up. That's assuming the battery would continue charging to a maximum charge when attached to the adapter. However, you could program the laptop to only charge the battery to 80-90% capacity, I suppose. My batteries live in the fridge at ~40% charge. Hibernating would be no benefit when attached to the adapter. It would use very very slightly less energy if any and it would take 50 times as long to wake up.I recommend you set it to hibernate, not sleep. Sleep mode still uses up power (albeit very little).