Actually, it depends on how you are setting things up with the magma crucible/thermal generator. You basically need 24000 MJ to get 30000 EU so it is definitely not worth firing it up with an electric motor that you feed with EU (best ratio is 2.5EU gives 1MJ), but then that is not really the point!
I have 3 combustion engines running biofuel hooked up to the magma generator (with cobble) then to the thermal generator. This keeps the magma generator running just short of continuously and gives me a lot more EU for the fuel than I could get otherwise (roughly 250000 EU per bucket of biofuel compared to the biogenerators 32000EU).
The 'infinite' energy solution is that you kick start the process with a bucket or two of lava in a magmatic engine and use that to power the magma generator with netherrack in the generator. The engine produces 18000 MJ/lava and the generator (with netherrack) uses 12000MJ/lava giving you a net 6000MJ of energy in lava that could be piped to a thermal generator. I assume you pipe once the magmatic engine is full of lava.
While one engine does not feed energy at the required tick rate the magma generator buffers up to 24000 MJ so the process just runs more slowly if there are fewer engines.
Anyway, that is all I know, hope it helps!
Currently playing: Minecraft, SimCity 4 (yes really)
Currently retired from playing: EQ2, Vanguard Saga of Heroes, Fallen Earth, WoW, Mortal Online, Perpetuum, Rift, EVE, SWTOR