This was tested on an asteroid base in 200.123. Asteroids have a 500-unit charge, by default, which is from the old power system. With new reactors placed on the asteroid, regardless of the reactor size, all factories pull from the same 500-unit power pull on each tick.
For debug purposes, it would be useful to add to the display module the power supply and power demand tags to see if power demands are being met of an entity that has no ship core (asteroid, planet, station).
It also appears that asteroids generate a small amount of power once a block has been placed on them... that was neat to discover, and solves my long-standing gripe about a no-gear start not being possible. Thanks!