The most efficient use of solar power is lighting. Sunlight is already light, no energy is lost in conversion to or from electricity. Thus the success of windows and the more moderate success of skylights. But What if you need the light to get somewhere not directly connected to the outdoors. What if there's three feet of insulation between your wall and the outside, as there probably should be.You could use expensive fiber optics to move the light around, as we've seen at Treehugger before. Or you could just shunt the light into a highly reflective pipe, and pipe the light into your house. Light pipes are not a new thing, but advances in inexpensive, extremely reflective materials have recently made them more viable. The people at Solatube, for example, seem to have a really great system going.
The cap of the light pipe redirects light straight into the tube no matter where the sun is, and then their proprietary reflective pipe transports the light into the interior will relatively little loss of light. At the end of the tube, a refractive lens or mirror spreads the light through the interior of the building. Their system has already been installed in factories, warehouses, retail stores and homes across America. No word on cost per installation, though I imagine it varies pretty widely. I sure would like to have one in this basement office.