[iwwr]

Space-based solar may work in powering space probes that are too far from the sun. An array of solar concentrators could beam down just enough solar power downrange to make the exploration of the Kuiper belt and inner Oort cloud worthwhile (there is no loss in conversion since you're redirecting photons all the way to the spacecraft). The alternative thing may be orbiting microwave relays that receive power from Earth and then beam it at the distant spacecraft (either as laser or microwave power).