Many mountain hikers recommend drinking hot fluids, because they
provide a lot of energy.
Not really. The ideal core body temperature has traditionally been stated as 37.7°C. We reduce human to jar of 80kg of fluid content. Imagine what happens if you pour 0.2kg of 45°C fluid into the "human jar". It is pointless. The drank mass and the available temperature difference is futile.
However, as @Chris H mentioned: the fluid goes directly to the core and its temperature could be higher than the 45°C. This makes the energy from hot drinks slightly less insignificant.
So it does not provide energy because it is hot. The most proper reason why to drink hot drink is "placebo" effect. Because of that and obvious security reasons I would suggest the following rule:
Aim for the lowest temperature that makes you feel warm.
What is pretty consistent with great answer from Marcus Wigert.
To obtain energy from hot drinks, it is suggested (thanks to @Chris H for pointing that out) to put directly a source of energy into the hot drink. In my personal experience, I would recommend combinations of following:
- unflavoured whey protein concentrate
- cocoa powder
- coconut oil
- honey (in extreme situations when immediate energy is required)