The quintessential free lunch
Sunlight...that's it folks. The harvesting and processing will cost you.

The Art of War
http://classics.mit.edu/Tzu/artwar.html
II. Waging War
9. Bring war material with you from home, but forage on the enemy. Thus the army will have food enough for its needs.

"But forage on the enemy." is the essence of the "free lunch".