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Plasma Reactors are the main source of power for any heavily urbanized area or big machines.

More primitive/smaller devices or areas use gasoline or similar types of fossil fuels.

I'd assume the Imperium uses coal/petroleum power for it's cities due to how presumably plentiful the stuff is on worlds with abundant plant life.Of course, for more power intensive machines, they'd use Fusion.

I'd like to think that necrons power all their tech with vacuum/ zero-point energy. "We're so advanced that nothing powers us." Or they could be able to transmit power from any point in the universe, the old book has references to a Dyson sphere, they could have a dozen stars caged up to extract all the energy.

The Tesseract Ark is a black hole containment device that can let it out in little bits as a weapon.