The other answers are good but do not take into account a certain striking highly legitimate/relevant scientific angle, have long noted the following but never seen anyone else point it out.
Thermodynamics and physics are certainly relevant/useful/compelling concepts to analyze this question (and the robotics in the movie tends to steer thinking/interpretations in this direction), but in some ways is misleading. There is a key concept from biology that is highly relevant:
In other words, in stark specifics, to mix biological and technological metaphors [but exactly as the movie does], a model for the movie is that its a bizarre robotic-human parasite interface, where the human race is the host and the robotic race is the parasite "species". This interface sounds weird but it is also the basis of the theory of "cybernetics" aka termed by one "the marriage of man and machine" (and related to the origins of the word cyberspace).
A relatively new book on this subject by Zimmer, Parasite Rex, documents how there has recently been a real/total Kuhnian paradigm shift (mass/systematized rethinking) in biology on this subject. parasites are now known to alter the behavior of their host in key ways. the parasite extracts energy from its host but not in so much an obvious way but in an indirect way (eg parasite feeding off host blood, which houses stored energy). also up to a staggering 3/5ths of all species on earth have parasitic aspects, that is, it is the basic mechanism (of life/energy extraction) employed by the majority of life on earth.
So far it is not thought that many parasites are "designed/optimized" (evolved) to focus primarily on human hosts [ie we believe we are at the "top of the food chain" as the saying goes], but one certainly wonders! A semi scientifically plausible candidate is as follows! the parasite Toxoplasma gondii is estimated to infect up to ~30% of the entire human population. (and how much does this ~2.5B host count compare to its other hosts, cats and rats?)
It cannot reproduce sexually in the human host but it can reproduce asexuallly in humans! toxoplasma can be transmitted vertically from pregnant mother to child!
T. gondii is thought to be mostly dormant in the human, although it is now documented to alter human behavior. it is implicated as a linked factor in schizophrenia. scientists admit that there is much unknown about T.gondii & overall study of it is in many ways still in early stages.
Another striking example/video of a parasite controlling the behavior of a insect host, the fungus parasite in rain forests, and thereby extracting energy almost as if the host is its battery.
Yet another key element that no other answers point out: human slavery has existed for countless millenia, probably as long as humanity has, and slavery persists to this day around the world. The most simple analogy is that the robotic species has enslaved the human race. Slavery is once again an indirect harnessing of energy of one individual by another. Of course there are also strong parallels between parasites and slavery such that parasites are said to enslave their hosts in almost all senses of the word.
Even in western civilization slavery is thought to be banished, but a more subtle but very similar form of wage slavery is extremely widespread and at record levels as revealed through statistics on economic inequality. These are somewhat more abstract analogies, but the economic system is an energy system and slavery is a harnessing/extraction of energy through the economic realm.
 Parasite Rex: Inside the bizarre world of nature's most dangerous creatures
 Absurd Creature of the Week: The Zombie Ant and the Fungus That Controls Its Mind