I was just looking at all of the replies to this question (looks like a
popular question). Ian D. Bruce was on the start of the most quantitative
answer. May I provide my semi-quantitative answer:
If you were sleeping, it would still take about 1500 Cal just to have your
heart, and other organs working. I don't know offhand how much energy
comes from the sun, but assuming that one can get 20mg of sugar per squar
decimeter of surface area per hour and that we have 170 dm^2 available on
our skin, we could produce about 41g of sugar or about 164 calories per
day (12hrs).
If we assume that leaves are about 10% efficient at converting light to
food, and we could do better (say 100%) then we may be able to live off of
photosynthesis. However, in general, biological energy conversions are
about 10% (so I learned). So our best bet would be if we could shut down
our heart and rely on evapotransporation for moving water and nutrients
around and stop our running to and fro -- but then we wouldn't be much of
animals.
I'm not sure exactly how precise these numbers are but the are in the
ballpark. Keep asking fun questions. (I will give credit to George
Ahlgren, my Plant Phys professor (a few years back) for the general ideas
and numbers in this answer)
Kent Johnson
73773.1320 at compuserve.comkentjoh555 at aol.com
