The interesting thing about investing in energy, is that you get a return on invested energy, just like you do on invested cash.
Build a few solar panels, and you have energy to build more solar panels, you drill for oil, and you have energy to drill for more oil.
At some rate, you can calculate the return of "free" energy you get out of of any unit of energy, just as you can for a return on invested capital.
Oil's return has been estimated at about 20-30:1 depending on where you get the oil, and how easy it is to refine into something usable. (i.e. one barrels worth of energy gets you 30, one of which has to be put towards the next 30 barrels, so you really have 29 barrels available for other purposes)
Coal has been estimated at about 10-25 depending and so forth.
wind and solar were estimated at about 5-10, but technology is likely making this a larger factor as we type.
http://www.abelard.org/briefings/energy-economics.php