Power cables show "not connected to a power grid" in the energy tab, although they are connected.
Doesn't matter what kind of power cable.
For batteries, transformators, generators and even heavy power connection plates/tiles the energy tab works as excepted.
In ONI Vanilla this tab for power cables work.
(Sorry for possible not exact english wording, I am playing with German language.)
Start new Spaced out game.
Build a generator or battery with some connected power wires.
Select any connected build power cable.
Switch to energy tab of the cable.
Watch it saying: "Not connected to a power grid".