I think that one of the larger issues with solar arises from the "maintenance-free" side of it:
Lets say that you need a large number of EUs for a project, or running a base. You have the option of either building a colossal nuclear reactor, and watching it for the entire duration of running, to prevent it overheating, or you could build a load of small reactors, which will run for a few hours, and then run out of fuel, giving you energy, but still at a fairly hefty cost.
Or, you can build a stack of solars, log them onto copper cables, and leave it for an IRL night. Plonk a Mass Fabricator on there, too, and you have a load of diamonds into the equation, too!
Now, I'm not saying that I have a big solution, but the main issue is arising from the ability to "fire and forget" the solar generators, which, as the most powerful renewable source, makes them OP, for the moment.
Perhaps one method would be to stop them generating when no one's in the vicinity? Or take them down to the levels of wind and water?