Joe, although batteries are rated by the power they put out, much more important is the quantity of energy they can store (expressed in gigawatthours). "10GW of storage" doesn't tell me anything about how useful that battery will be to supply energy to the grid.
Today on CAISO at noon we had 3800MW coming from utility solar, with 6.3GW of capacity installed (5.2GW as of 3/19/2014 - I'm adding 550MW each for Topaz and Desert Sunlight now online), so at noon today the efficiency of California utility solar was 60.3%.
To get 15GW of actual solar generation at noon you'll need 25GW more solar capacity (multiplied by .6 = 15). That's equivalent to 45.5 more farms like Topaz or Desert Sunlight at a cost of $90 billion at today's prices, and occupying a land area of 440 square miles (equivalent to covering every square inch of San Mateo County with solar panels). The price might come down somewhat, but that doesn't matter - with the storage you're adding in, it's by any measure a ridiculous amount of money to pay, and a ridiculous price in land use, for energy that comes and goes. And it all has to be backed up with fossil fuels.