There are a couple of stories where humans gain chloroplasts.
Here's a clip from one story:
They are also infused with bacteriorhodopsin, allowing photosynthesis. It is now possible for a human being to lie on the ground, in the sunlight, for thirty minutes, and absorb all the energy and nutrients they need for a 24-hour period.
But, this strikes me as unlikely. The other story had humans being powered by sunlight.
While working with this idea we happened across an interesting and rather common tittle freshwater coelenterate named Hydra viridis. This particular hydra, unlike its relatives, exists in a symbiotic relationship with a wonderful alga called chlorella.
In addition to producing its own food, it will also produce enough to keep its host alive. "When chlorella is not present, the hydra is forced to eat solid food just like its white relatives."
...will have to believe you after you've done nothing but sit in the sun and starve yourself tor weeks on end without getting hungry or losing any weight.
EDIT: removed 3 hours a day for power/meals, since the story didn't clearly say that, only implied it.
It sees to me that an animal running around needs a lot of sugar/energy. Even if we take out the energy needed to process food (a non-trivial amount), it seems to me that a human would require more power than could be provided by merely having a green skin. Square-cube law, and more mass behind a smaller surface area than a microscopic animal.
IIRC, humans need about 2,500-3,000 calories a day.
Working skin is a under a couple of layers of dead skin, which should impact how effective the chlorophyll would be (how much?)
Assuming there was a way to prevent them from being liquidated and absorbed; how much energy would a bunch of chloroplasts within vacuoles in the human skin provide?
(an aside, too bad the fortnightly challenges don't say when they expire)
Hmm, the calorie consumption numbers are for food, which humans are not good at digesting, does anyone have glucose/ATP consumption figures for human beings? Maybe how much are people who're on IV drip fed?