Short answer to that is that even turning the entire skin into a chlorophyll bearing membrane doesn't produce photosynthetic energy efficiently enough to power a human being.

That was part of the point.

The long answer is the same as the above....BUT we can multiply the efficiency of photosynthesis by using an much larger external photosynthetic energy membrance and consuming the energy produced....you know, like eating plants.

And if you want an even more concentrated source, you eat the animals that ate the plants.