A watt is a unit of POWER. A watt-hour is a unit of ENERGY. (A calorie is a unit of ENERGY. But the figure you cited should actually be expressed in KILOcalories - a common mistake; the food-energy values we all know are actually KILOcalories.)
One watt-hour is equivalent to 859.8 Kcalories. So 300 Kcalories are equivalent to 2.87 watt-hours.
Thus, if the brain consumes 300 kcalories per day, that would be equivalent to 0.287 watt-hours per day.
In short: Your math is faulty, and you are also using incorrect units.
The correct figure is: The average human brain - when at rest - consumes approx. 287 watt-hours per day.
The amount of energy the brain consumes when actively thinking is not much greater: That's because most of the brain's energy output serves merely to keep approx. 3 lbs of living tissue warm and alive.
Regards,
All right. So, how much in power, as far as cost goes, is the brain using?
In my province, that’s about 5 cents. Still very efficient.