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.)