I'm having trouble figuring out this particular piece of game mechanics.
I get it that energy is consumed by construction as well as by fighting.

But in this particular case (mission 8) I haven't built anything but power generators that I used to take over some of enemy's power generators. Initial energy reserves were suitable for maintaining shield on border power generators in order to capture enemy's generators as well as protect mine from being taken over.

However, just as I was thinking everything was going well, my energy took a steep dive and *never* recovered. I did order building some labs because I was going into offensive mode, but even killing those labs didn't help at all.

So, where is all my energy going?

is it biodamage or krugh mission ?

krugh

Yeah, thanks, just figured it out. Enemy built howitzers and was killing my structures. Repairing took all the energy. Seems I have to build my howitzers first :)

BTW: One more energy-related question:
The power generators raise up this "envelope" around their towers and when it's all the way up, the generator can raise the shield.
What is needed for this "envelope" to raise in the first place?

each generator once is build need to be charged to be able to raise the shield, you can select it and when is fully charged green, you can raise the shield. I hope that helped