Has never been measured, but is calculated using about 100 watts.
Has never been measured on purpose, but logical deductions math that may be acceptable.
All neurologists agree that the brain human consumes, on average, 20% of the energy we spend in total. It is also believed that an adult “works” with about 100 watts, which it follows that we spend thinking about 20 watts.
These calculations are to convert calories we eat in joules (work), and that unit watts.