Never meant to imply that your ultimate conclusion - that the brain is efficient - was incorrect. Was merely addressing what I viewed as faulty math and incorrect units.
One could answer your question - "How costly is the operation of brain?" - in a different (I think: more elegant) way:
We have established that the brain uses about 300 kilocalories per day. But the brain is not powered with electricity. Rather, ultimately, the brain oxidizes glucose to generate energy. Glucose has a current market price of approx. US$4-6 per kg. 1 kg of glucose represents 3,860 kilocalories. So the brain requires approx. 80 grams of glucose - worth some 31-47 cents - per day.
Bear in mind: That's for a brain at rest. But as I said in my previous post, the remarkable thing is that, even when engaging in intense conscious thought, the brain consumes only very slightly more energy. I haven't looked it up, but I'd venture to say that it never consumes more than, say, 5% more than at its baseline.
So, in a way, it's also very inefficient, since it's kind of like a supersonic airplane that consumes 95% of its maximum fuel requirements when just sitting on the tarmac, and only 5% more when flying at Mach 2.
Another thought: When some people eat, say, a candy bar "because it boosts their brain-power / helps them think" - it may, in reality, simply be that they are suffering a bout of low blood-sugar, which is also affecting other body functions like the blood pressure, which in turn affects the brain. So I don't think that eating a candy bar DIRECTLY helps the brain / supplies it with more needed glucose. The body always gives the brain top priority when it comes to nutrients, anyway. But if the rest of the body is somehow starved, the brain will, of course, also suffer indirectly.
Anyone: Please feel free to correct my thinking if my figures are off here! As for my other speculations: I'm just spitballing here!
Regards,
Thank you for correcting my reasoning.