If you accept the numbers at http://www.theoildrum.com/node/6252 (which are sourced, but that doesn't make them necessarily accurate), it takes 0.430 kwh of energy to produce 390 food calories (390 kcal) worth of corn, for an efficiency rating of 102%.
I thought this was impossible at first, since it would seemingly violate the Seecond Law of Thermodynamics, but the calculation only includes the light energy required: the additional caloric energy comes from the physical matter that makes up corn, primarily carbon, hydrogen, and oxygen. In other words, creating corn requires both energy and raw materials, and the raw materials themselves contain stored energy.
Assuming you could live primarily (though not entirely) on corn, this number is close enough to 100% that we can assume roughly 1-to-1 transfer of light energy to caloric energy.
The average healthy person burns at least 2000 calories a day (right around 100 watts), though this assumes very little activity. With a reasonable amount of activity, this is closer to 3000 calories (about 150 watts).
So, somewhat uninterestingly, the amount of energy you need is, assuming you have sufficient raw materials, is pretty much the amount of energy an average person burns.