Basking or working in a hot sun requires an expenditure of bodily energy. You get hot and start sweating, breathing more and, fanning yourself. In addition, you are frequently more active on a day in the sun. So bodily energy goes out, not in. Charging a battery means you put more energy in than comes out. The amount of energy absorbed by the body is only thermal heat, of no nutritional or stored energy value. Different algebraic sign. There are some effects, for example, manufacture of vitamin D by the skin but these are probably negligible energetic value.
By the way, I agree with you. I am frequently tired after a day in the hot sun.
It actually helps to distinguish between total
energy and free
energy, a quantity that involves both energy and entropy. When you're hotter, your total energy goes up. The ability to do things, like move your muscles, requires free energy. If you happen to be able to photosynthesize, going out in the sun might energize you, just as a battery left in the sun will charge up if it is hooked to a photocell, but not by itself. I did know someone in college who believed that he could photosynthesize if he tried, but it didn't work out well.
(published on 07/05/2009)