Solar thermal power plants in the desert and on factory roofs can generate power all night long, after the sun goes down.  How so?  With heat stored in molten heat storage salt or in factory products like cooling glass or metal.  With cogeneration, a turbine generator running on constantly rec-condensed, recycled refrigerant gas heated by the stored waste heat.  Red hot metal or glass stores a lot of energy.

The really large loads on the grid come from heating/cooling buildings.  Heat/cold are easily stored in building mass.

Backup power can come from biogas employed in solid oxide fuel cell/turbine power plants distributed around the grid.  The ultimate backup source can be natural gas, decades of extra supply exist in the form of coal, oil sands, and oil shale that emits natural gas.  That natural gas conversion can be increased with human help.

And solar is not as intermittent as it appears.  Thermal solar furnace powered factories that refine silicon or recycle glass or metal, for instance, can cogenerate power all night long from the heat of the products.

Wind distributed widely around the continent, connected to a High voltage DC (HVDC) grid provides a stable baseload power source.

A smart grid can store energy for high load uses in building mass, emergency power levels for communication and lights can be augmented with batteries for short term storage, 12 hours emergency power to get through brownout, black out and storm incidents.  

Natural gas/biogas backup generation distributed in local areas on farms, landfills, and sewage processing plants would be enough to power the grid for longer emergencies.  

The main cause of power emergenicies is not peak load, it is storms.  Storms growing ever worse seemingly related to GHG cklimate change.  More solar energy trapped in the atmospheric system, more and more severe storms.

I think a different view of the grid as a self survivable system that manages demand as well as supply with renewable distributed generation and storage is necessary.  The old monolithic central generation grid is obsolete.  It is killing itself with GHG climate disaster.

The more central power, the more GHG, the worse the storms, drought, and heat events that break it's back.  No way it can get more reliable.  It will only get worse.