The way to do this is to solve this logically: going through the options one by one and eliminating the ones that do not work.
- NiMH and NiCad: Great for small things, but powering a whole planet could get a bit out of hand
- Lithium Ion: Just the one?
- Hydrogen fuel cells: Hydrogen is not even an energy source!
- Alkaline: Similar scalability problems
- Zinc / Magnesium: Needs other things (such as lemon juice).
- Methane: A good supply due to cows farting, although collection could prove difficult.
- Lemon juice: Sure, why not? You'd just need lemons the size of a small moon (and a plate of zinc to match)
- Wind (Not including Methane): Environmentally friendly and cheap but disliked by a lot of people (they don't look nice!) Then again, since when have Perl programmers cared about things being pretty? ;)
- Sunlight: This presumes that we don't move far away enough from our Sun that we can't get enough watts.
- Tidal forces: Ditto, re non-Earth.
- Diesel: More energy than petrol, this may in fact work (if we build enough machines to counter its side-effects)
- BioDiesel: Possibly! It depends if it catches on enough or not.
- Gasoline: We'll either run out of it or develop something better in time
- Natural Gas: Supplies will run out before it gets the chance to run on everything.
- Greenhouse gases: Ditto
- Camels: Camels can turn rotary wheels without needing much of their own energy. Oh, and they can have a nice desert where they can roam around in and code Perl. Hooray!
Personally I think that it'll be some sort of nuclear. Fusion power will be invented in about 50 years if SimCity is correct. After that, a matter+antimatter combination, when it becomes readily available (don't ask me when)