The main power loads are during daytime. So solar is a pretty good match to that, especially, if you like the US have large desert regions, where weather related changes are unlikely to reduce the solar efficiency a lot. Add to that wind power, which runs around the clock, you have a pretty good cover of the load. Of course you need also storage. That can be batteries on the small scale (also a good way to reuse old electrical car batteries), and hydro on the large scale. Just limiting the Hoover dam to produce electricity as a backup to solar should cover quit a bit of the storage requirements. If the remaining gaps are filled with natural gas, its a good balance. Natural gas produces much less CO2 than coal. And if only 10% of the energy load is done with gas, we should no longer worry about the CO2 too much.