What is the energy consumption of the brain?
I assume that you mean energy consumption.

Talking in round numbers the daily energy requirement of a sedentary human is 2,500 kcal. It's usually said that the brain uses 20% of this energy.

What does this mean in terms of power consumption?

500 kcal = 500 x 1000 x 4.2 J  -[1]
24 h = 24 x 60 x 60 s          -[2]   

1 W = 1 J/s

therefore brain power  = [1]/[2] = approximately 24 W


Most of the energy consumed by the brain is used in maintaining membrane potentials so energy is used continuously. I would think that there will be very little difference between waking and sleeping.