Airbus brake fans are just controlled by the pilot by a switch, not automatic at all (at least on the 320 series). They are electrically driven. I don't know whether you can switch them on in the air, I have only flown a jet once with them fitted.
A320 has 3x 90KVA generators, one on each engine and one on the APU. I would guess if only one was running it would be around the 50-60% load.
Tom.