Build four LV capacitors. Place them down, not interacting, in pairs, with the two capacitors of each pair 16 blocks apart. Fully charge one capacitor of each pair while leaving the other empty. Connect one pair using IE LV connectors and IE LV wire; connect the other pair using IW copper connectors and IW copper wire. Allow the full capacitors to drain into the empty ones.

At the end of my test doing this, the capacitor charged via IE LV wire had 95311/100000 IF in it, while the one charged via IW copper wire had 95246/100000 IF—almost exactly the same amount, and a loss of just under 5%.

Now make an Engineer’s Voltmeter and shift-right-click on the connectors of the two pairs. With the IE LV wire, the voltmeter will claim a loss of 5.000%. With the IW copper wire, it will claim a loss of 0.156%.

Minecraft 1.12.2, Immersive Engineering 0.12-98, Industrial Wires 1.7-36.