I browsed the suggestions but didn't find anything like this, so tell me if I'm wrong please!I like to make stuff look nice, so, I usually try to hide all my wiring, Power Units, etc...
But then I find myself running and checking my MFS unit to see if it has enough power to run the Mass Fabricator for a while, etc...
So I suggest a box that you connect to anything that has contains EU (I suggest having it use Tin Wires (super low voltage), and have it show the amount on the Display. This way I could run all my boxes to a control room where I can keep tabs on the energy levels of everything.
Alternatively, when you connect this box up to a Nuclear Reactor, it could show Heat.