I'm guessing you'd need some huge surface area to power a human with photosynthesis.
Perhaps an acre (wild guess)? Maybe someone could look that up?
Perhaps an acre (wild guess)? Maybe someone could look that up?
Gives 1538 Calories per pound of whole wheat flour.
According to these folks an acre of wheat yields 42 bushels: http://www.wheatfoods.org/AboutWheat-wheat-facts/Index.htm
At 60 pounds/bushel that gives you 2520 lbs. * 1538 Calories = 3875760 Calories / acre / year. We'll divide by two to assume you're mixing wheat with some foods that are a little less calorically dense, because you'll want at least some green leafy veggies and beans for iron and protein, and that gives us 1937880 Calories per acre per year, divided by 365 days in a year yields 5309 Calories per acre per day.
So, back to the original point, it looks like you can easily power a human using half an acre of land and photosynthesis. Possibly one quarter of an acre if you're willing to live on bread alone. (Whole grain being not so unhealthy as white.)
That said, you tend to lose a ton of efficiency if you grow a plant, cut it down, process it, and then eat it. That plant is doing a lot of things other than making you food. If you are producing your energy straight from the sun, you'd do this a lot better. I'd say you could get away with far less than your estimate.
Whenever I do these kinds of calculations I'm always amazed at how much power the sun is able to provide on a square meter 93E6 miles away!