Nitpick, I know, but: it's not powered by gravity it's powered by human muscles, so in the end - by food.
- Human muscles get energy from food.
- Food is or gets its energy from plants.
- Plants get energy from the Sun.
- The Sun's energy comes from gravity-forced fusion.
So gravity :)
the FLRW model of cosmology, which has made numerous confirmed predictions teaches us that the big bang is a geometric property of space-time i.e. a consequence gravity, and particularly Einstein's field equation from general relativity.
It's interesting to put that in perspective: the pyramids were built out of solar energy by way of muscle. Skyscrapers are built out of solar energy mostly by way of diesel fuel.
No wonder we're seeing exponential growth with that injection of stored energy into society. The industrial revolution is a coal-powered nitro.
https://www.youtube.com/watch?v=N1pIYI5JQLE
He explains the light of a fire is just the light of the Sun, it's just been stored in the wood
The kinetic potential battery (the weight lifted against gravity) is also very efficient.
Food is very energy dense. https://what-if.xkcd.com/128/ (sandwich vs battery)
Uh, change of topic: isn't the caloric value of a food how much heat can be released by burning it (probably when it is dehydrated)? Plenty of energy stored in a sandwich, sure. But a car battery could power a small spaceheater-like thing for... a while. There's a lot of potential for heat production in a car battery (even without BURNING it). Did Mr. Munroe get this one wrong?
> The efficiency of human muscle has been measured (in the context of rowing and cycling) at 18% to 26%. The efficiency is defined as the ratio of mechanical work output to the total metabolic cost, as can be calculated from oxygen consumption.
Similarly, I think saying both 'the sun is powered by gravity', 'the sun is powered by hydrogen' and 'the sun is powered by fusion' is reasonable.
this is because if something is collapsing from gravity it will stop once some other force is pushing out harder.
this explains a lot of phenomena and is a key piece of understanding the modern theories of stellar phenomena, e.g. via the pauli exclusion principle, why neutron stars can form during a supernova (from the extra inward pressure combining with the gravitational collapse exceeding the degeneracy pressure of electrons in the plasma)... its also why beyond a critical mass the collapse doesn't stop at all and produces a black hole.
in any case energy is always conserved... but the ultimate place it is being extracted from to power a star is the gravitational field. everything else, e.g. the nuclear fusion, the light pressure from temperature etc. is a result of that energy being transferred into the star as it forms and continues to evolve.