I wonder if the energy delivered to brain per unit time to maintain brain processes is limited and fairly constant? Is all provided energy fully used?
There is very little change in the overall brain energy metabolism with task.
This question has been answered previously at Biology.SE: https://biology.stackexchange.com/questions/839/how-does-the-brains-energy-consumption-depend-on-mental-activity
The top-voted answer there cites this review:
the local energy consumption increase owing to a typical task-related response could be as little as 1%
Although this may not seem intuitive, it does start to make sense when you consider the way energy is used by the brain and how brain activity changes over time. Active processing results in changes in patterns of brain activity: which neurons are active, not which regions. There is always a baseline level of activity happening in the brain, even in areas not involved in the current task.
Additionally, most of the energy used to support neurotransmission is by the sodium/potassium pumps that maintain ion concentration gradients. These pumps are always active, working against passive leak as well as neuronal activity.