The San Francisco Chronicle is reporting that the California PUC
proposed requiring the state's utility companies to buy more than
1.3 GW of electricity storage by 2020.

The 1.3 GW capacity is only half of the information needed.  We
also need to know how many minutes or hours or days the storage
will run at the 1.3 GW level.

The cost of capacity is about $1 per watt so 1.3 GW represents
about a 1.3 billion dollar investment.  However, the energy that
can be delivered is an additional cost.  Another 1.3 billion
dollars of energy storage (batteries) would buy you a discharge
time of about 2.5 hours at 1.3 GW.  If you wanted enough energy
for a whole day you can see this would be very expensive.  This
cost of batteries is the main problem of making wind renewable
energy a reliable source of power.