Problem is intermittency.  When wind stops blowing and its cloudy there needs to be 100% of the load's power requirements to be met by traditional sources.   And when there's insufficient traditional back ups due to excessive reliance on interruptible sources. the blackouts will occur.  And wasn't helped by high demands of a heat wave, recently. 

Another complicating factor is that base load plants cannot be started in just a few minutes. Takes a while to build up temperatures, and steam to turn the turbines, whether in state or out of state suppliers. Yet the wind and solar can stop in minutes. 

Also they all have contractual agreements with neighboring utilities to supplement supplies from those additional utilities, (when power sources are available.)

And of course, for many years essentially all utilities had been using load shedding agreements with some large customers (some with their own standby units).  Also companies would use some voltage reduction techniques.  Their final, reluctant step of last resort is rolling black outs

A while back I read opinions where some have said at about the levels as low as 10% or perhaps stretching up to 20% from intermittent sources, the avoidance of load shedding or blackouts becomes pretty risky.   

But the media are pretty good at politicizing the issue.   And hardly any sources have publicized the list of California traditional source plants that the state is scheduling to be closed.  A significant cause for concern regarding blackouts, is plant closings.