Solar Power Satellites

If you use laser power beaming you don't need to build it large - the
large size is needed for microwave transmission to collimate the beam
over those long distances. Lasers are less efficient, but it could work
well for a lunar base. And the receiving station would just be solar
cells which would be useful for extra day-time power as well. Of course
this works best for a base near the lunar equator that has to survive
long sun-less nights - the polar bases NASA is planning now probably
wouldn't need this, though it might help.