IIRC ( and it was 20 years ago now that I learnt this) the brain uses 20% of the body's resting energy usage. Most of that is keeping neurons polarised to the outside (ion pumps need ATP!!!).
The body uses 25w resting and thus the brain is about 5w.
Source: biology degree but like I said please take with the same amount of weight as a hallucinating LLM.
GPT says: Unless you're a hamster hooked up to a Fitbit, it's more like 60–70W for a normal adult human. So the brain’s real power draw is more like 15–20W, not 5W
Resting energy usage in humans is ~1200–1500 kcal/day, or about 60–70 watts, depending on the person. Logic holds, estimate is just low