Quick google search is about 100 wH per square foot of surface area. I'm just estimating about 8 square feet of surface area (one sided) for a human. Assuming 12 hours a day at that rate you get about 9.6 KwH or ~8000 kcal. With perfect efficiency and a storage system inside the body (which we do have, but not for light energy) that's probably doable.

Obviously our tissue is much well less adapted to so many hours of sunlight compared to plants. Also even plants only absorb a relatively small part of the light spectrum (none of the green light). Also even plants required compounds from the earth and the air to actually harvest the energy to store so it's a pretty complicated process to do in an actively moving human.