Seriously, I must ask this. Solar energy is derived from light. Any light. That's why solar calculators work well in office buildings. It would be a viable power source even in underground lairs since glowstone and jack-o-lanters give off the same amount of light as the sun, torches run a close second. Light is light, even if not generated by a ball of burning gas.
I love to live underground and sometimes it might be nice to have an alternative to geothermal and water. Sure, solar is expensive but it's also zero maintenance. Properly set up it's also an excellent source of power. Even if the power output was reduced as a penalty for using artificial light it might make life easier on those of us who live like hermits.
And, since I don't want to make a separate post about this unless I have to, is it possible to heat up a feeder reactor using the heat given off by another reactor? Essentially making 2 feeders heat each other?