Humans can use more power, they're just a lot more limited in their potential. Michael Phelps gets up to like 400 watts per day while at peak training.

2500 kcals/day would be around 121 watts, yeah.

But computers have much lower idle consumption than humans. So if we assume alphago idles at ~45w that would consume around 1000 kcal for 21 hours leaving 1500 kcal for the 3 hours it was given to think which would be more like 580 watts.

That sounds pretty doable.