Main power source is the Sun. This is one of the main reasons that space settlements are in the open space and not on planet, asteroids or other objects. Through clever calculations one can make that the settlement is always in the Sun.
Thanks to enzymics, the energy can be stored it can be as carbohydrates, who, if needed, can be eaten by humans.
For for the rare prolonged missions in the shadows of big stellar bodies as planets and moons energy generating midas devices are used.