The problem of the mass fabricator is it will suck all the energy out of your power grid. So you can't use your machines anymores.
here is a little circuit that give to the mass fabricator only the surplus of energy.
your need to use some mfe or mfs, depending of your production capacity and adapt transformers as well to not vaporize your base. if your production is less than 128 eus mfes are just fine.
-> you supply two mfe in a parallel circuit, the second is redstoned. so he will only output his energy if he's full.
-> the first mfe supply the second one and the third as well
-> the output of the redstoned mfe is splitted. one branch supply the mass fabricator, the other feed the third mfe.
-> the third mfe feed your machines and uses the energy of the first and the second mfe. it mean if your machines use more than your power source, your mass fabricator will stop working.
what do you think of this ? i just tested it, it's works like a charm but it's a bit expensive. a 128eu/s is do-able. but the mfs setup will be very expensive did you find a better (cheaper?) way to use a high energy input without any waste ?
there is an other problem, you need to fill complety the redstoned mfe before you can do anything with your power gris. Again, it's easy with a cheap setup but pain in the *** with the huge capacity of the mfs units ...