The only thing I can think of that would produce enough power for a suit would be a solar reactor.
Essentially the Solar Furnace would be a container that houses a man-made dying star.
The energy emitted from nuclear fusion could then be used to rotate a flywheel, which would generate electricity to power the suit.
Since the nuclear fusion couldn't be stopped, and the reactor wouldn't stop producing power until the star's eventual collapse, electricity would be generated indefinitely, therefore making the suit immune to EMP type weapons
