Erm, two things there Quantumwolf...
1.) We do know how much a battery box holds and how many joules a smelting operation takes, it's written in the guide
2.) All of this is irrelevant in this test because you simply want to test for energy loss. If the directly attached furnace smelts more pieces of cobble than the furnace attached by cable, you have energy loss. If they both smelt the same number of cobble, you don't.
But, I think I know where your confusion comes from. You are expecting that when you input a finite amount of energy into the cables and measure it, that it would decrease slowly over time because of energy loss. This does not happen, however, because there is not actually any energy flowing through the cable.
What you measure when you read out the voltage in the cable is how much energy it stores. Each cable has a small energy storage ability, called self-capacitance, and if that is filled to for example 80%, you will be seeing a potential of 80 V. In this way, the cable acts like a tiny battery. And this internal energy will remain where it is, until you connect a consumer. It is not flowing anywhere. The cable can sit there for a hundred years, and it will still be at 80 V. Just like a filled battery box does not get less filled over time.
What I mean with energy loss, on the other hand, is that you get out less on side B than you input on side A. It's a
distance based loss, not
time based, and it only happens when energy is actually flowing from A to B.