>From a technical and high-level economic point of view, solar should be 'overbuilt' so that it supplied excess energy a decent percentage of the time, to make up for when it doesn't output near 100%....

> Either they're actually storing the energy somehow (which is something that likely should be rewarded), or ...

Storing it is the big problem. You can't just overbuild solar power and put the energy in the network. You need to meet the demand. ND that is problematic if you must mix and match different energy sources across the network.

You can actually if you have energy trading between regions. The problem with California is that during peak-power use times (18:00->20:00) there is no where to buy more solar from. But California can sell its solar for other regions peak-power use time.

I think the problem is that the US grid is also not well set up for this to be viable though, but I don't know the specifics (I think the east coast is on a different grid from the west coast, and Texas has its own grid?)