This site may help you.
http://www.energyquest.ca.gov/story/chapter01.html
I do not fully understand this question...
But just out of experience would you say that plants vary a little or a lot in their efficiency at producing joules for human consumption. And could you suggest reasons for the variation?
2 answers
Hmmmm. Some plants produce a lot of cellulose, and humans cant digest any of that. Plants produce oils, sugars, and some protein, and humans can digest those.