[SPOILERS] What sustains Simon?

[Abion47]

It's the battery that sustains him, yeah. It's the cortex chip interfacing with the structure gel that allows him to "live" and operate.

And actually, the amount of electricity required to run the human body is less than you'd think. This article talks about how the body is only able to generate 10-100 mV at any given time, and while I can't find anything on the amp equivalent to what muscles need to contract, there are already machines that artificially contract muscles which draw anywhere from 700 mA to as low as 10 μA, some of which that even run on AA batteries.

Also keep in mind that the amount of energy used by the human body is more focused by far in all the faculties that keep us alive - 20% of the energy we expend (on average) is just for the brain. Simon's robot body doesn't need to worry about operating the brain, heart, digestive systems, or any of the other energy-starved automated bodily processes. All it does is operate the muscles necessary to translocate.