Energy is just one limit for space travel. The other one is reaction mass — the matter that comes out the nozzles of your spaceship.

In our solar system there are 2 or 3 places where gas reaction mass can be scooped off moons or planets.

If a way can be found to bring liquid matter that would be even better because. Liquid has more mass than gas.

The energy source could be matter/anti-matter annihilation (E = mc squared).

A centigram of anti-matter + reaction mass = lots of thrust.