I know that in 10 man Onyxias energy gain isn't affected by how much dps you have on her but for some reason we've come to the conclusion that we need to keep dps Onyxia in 25. Does she gain energy faster if no one dps:es her?
I don't think its that Onyxia's energy goes up faster when nobody DPSes her but rather when Electrocute is cast by Nefarian's Lightning Machine it shoots her charge up a large amount. Watch it the next time you fight her.
The energy gain increasing with little or no DPS on Onyxia is very noticeable, even in 10-man. With our add strategy, we had to split up the remaining DPS to keep her from exploding before we forced two Electrocutes, as she was gaining just under half her charge bar just from passive growth (with the other half being from Electrocutes).
I haven't watched it but I know that we've been forcing 3 electrocutes in phase 1 by bringing her low then blowing up nef, her death to coincide exactly with the 3rd electrocute (so she doesn't blow up the raid) and we can get the raid topped before lava shows up. This is on 10 man, though. For a good minute of it, there's nothing but our prot warrior DPSing her.