as far as i know no cannon information has been given on ultrons power source, this leaves us with five possibilities (listed in order from most to least likely). (given that ultron was plugged into the internet remember )
- he used an arc reactor
pros: Arc reactors are as far as we know the most energy dense thing available on earth and can be made with relative ease (provided you know how, i.e. tony stark in a cave), arc reactors do NOT need to be visible to work, i.e. the hulk buster armour which had eleven only one of which was visible.
cons: it seems to be antithetical to ultrons character i feel he would try to distance himself from stark and prove his superior intelligence rather than just steal his fathers creation.
- he used chitauri technology
pros: he did use chitauri technology in other elements of his suit, such as his anti gravity floaty abilitys, chitauri tech is probably one of the few things easily available to him that could rival stark.
cons: chitauri technology has been implied to use elements not found on earth making it hard for ultron to replicate it.
- he invented some new technology wholesale to use in his designs.
pros: he was incredibly intelligent and scarlet witch did pull out an unrecognisable "heart" from his body (though we have no way of knowing that was the power source) much of the rest of his bodies were new technology. agents of sheild also has a bunch of werid science that could be used in such a situation like "darkforce".
cons: making whole new energy sources from scratch is hard. no one whatsoever has reverse engineered ultrons technology and created a whole new power source revolutionising human living. like with all thoose bodys lying around you'd think someone would figure out that a lot of technologically advanced stuff is in there right?
- he used some other human technology.
pros: he did access the internet.
cons: given what we see on screen there are only a few technology that could to that specifically: Nuclear, Radiological and Beam power it wasn't Nuclear beacuse stark doesn't remind everyone that "hey every time you blow up one of thees dudes sokovia won't be able to grow crops for another three years" nor did black widow get cancer while getting close and punching away. Beam power also seems unlikely as the avengers did not attempt to destroy some big relay station to kill all the drones. same anti-human arrogance
- he used the same technique hydra used with the tesseract, just with mind stone instead.
pros: we know that infinity stones are increadable sources of power. ultron would probably find it poetic that his source is what still gives him strength
cons: we don't know if its possible to do that with the thought stone specifically, only Zola seemed to have any success with that. the thought stone was eventually lost by ultron and he didn't seem to mind much. (no that was not a pun, as it would be a bad one)