The 57,600 MWh figure is the peak usage in California on the hottest of summer days. We only need that for roughly 2 hours of one of those days, and we can typically meet that without engaging rotating blackouts. The generation capacity is there. And it’s a lot more efficient to operate plants as needed to recharge storage than to spool them up in response to demand, the Tesla battery farm in Australia has already proven this. The key reason is there is lest waste if you have storage as a buffer.

And think of all the CA plants sitting idle on a day like today, where peak load was 28,000 MW.

http://www.caiso.com/Pages/default.aspx