The Remnant rely on radiothermal generators, because of their distaste for the unstable nuclear reactors used by early humanity. In addition, the Remnant mine crystals from Viminal, some of which can be utilized as batteries.
Remnant energy cells feature excellent heat efficiency. However, they are considerably less space efficient than most other reactors in the game—even human fusion reactors.
The Millennium Cell can be useful when one wishes to fill in that last bit of outfit space.
Crystal Capacitors are among the most space efficient batteries in the game, beating out the Coalition's Battery Modules and surpassed by only the QuargNanotech Battery. However, this space efficiency comes at the cost of a small energy drain and heat production, as per its description.