"Elaan of Troyius" puts the main dilithium crystal in the middle of the engine room. If the nacelles were self contained, that would make no sense. Ditto the antimatter pod Scotty was messing around with in (I think?) "Lights of Zetar", and the times Scotty glances at the glow behind the mesh in engineering to guage ships' power
It may have been the original intent that the nacelles themselves be the power source, but even if so, that's not how it ended up on-screen.