I found the energy usage by mass for the heart, kidneys, brain, and liver on Google Books. I got the masses of those organs from wikis and such. The result:
kilocalories/kg/day | kg | kilocalories/day | |
---|---|---|---|
Heart | 610 | * 0.3 | = 183 |
Kidneys | 390 | * 0.3 | = 117 |
Brain | 296 | * 1.4 | = 414 |
Liver | 290 | * 1.5 | = 435 |
So it's liver FTW!
Of course, there's error in those numbers, and each will fall somewhere in a range, but still. Quite unexpected.
Aside: Those numbers mean ~40% of the body's energy is used just by the brain and liver. ~55% by those four organs.
Aside #2: That Google Books link had another interesting fact. The brain in a newborn baby uses 74% of its energy!! Wow.