Human brain weight is about 2% of body weight. However, its energy consumption is 20%. Thanks to brain imaging techniques (like fMRI and PET) scientists can see the changing metabolism and brain blood streams, which mean energy consumption as explained by Marcus E. Raichle from Washington University . According to Raichle, only between 0,5% and 1% of the energy is used to bear with external world. 60% to 80% of the energy is dedicated to maintain the connection between neurons. Therefore, the creation of new neuronal routes is the main energy consumption mechanism.
Other source of energy consumption could be the constant prediction processes present in the brain. Human brain is often defined as a prediction machine, as it collects information and experiences from the past and tries to prepare and anticipate the immediate future.
 The Brain’s Dark Energy. Marcus E. Raichle. Science 24 November 2006: Vol. 314. no. 5803, pp. 1249 – 1250. DOI: 10.1126/science. 1134405