I think Data was talking about the M/A-M reactor system is generating 12.75 EW per second, (But the Alarm cut him off).
There's no mention of how much energy the Warp Nacelles consume at what speed, that's not stated anywhere.
There's no mention of Power output per Nacelle, and Nacelle generally don't create power, they consume it.
It's the M/A-M reactor system that makes power and the Nacelles consumer power when in use at FTL/Warp Speeds anyways.
No mention of conversion from ___ grams of Anti-Matter Collided to Energy ouput.
There's no mention if 12.75 EW per second is the Minimum/Maximum/Average power output of the Enterprise-D's reactor.
The only data I can gleam from that statement is that at that moment, the Enterprise-D's M/A-M reactor could generate that much power in one second.
That's what I got from the dialogue too.
That the Warp core itself was generating 12.7 Exawatts per second at the time... that's in the state that's associated with 'regular' ship functions while not at warp.
As for the Warp nacelles... the nacelles (as far as I know) only generate the Warp field around the ship (hence the presence of Warp coils in the nacelles) and consume energy produced in the Warp core.
How much energy each nacelle consumes WHILE at warp is not stated (as you said)...
Since Data's statement said 'we are PRESENTLY GENERATING...', this could imply the amount of energy in the core can increase while Warp drive is in use.. or, it could be the maximum of what the core is capable of - but that would seem rather wasteful, because it would be far more efficient to have the core produce less energy while say Warp drive is not in use... similar to how a computer would draw less power while its not being stressed.