Some numerical thoughts - a model:
Assume a requirement of 1MWHr/day avg. base load (+/- inclusive of variability )
Assume the windfarm operates at 33% efficiency/load factor ( wind variability and absence )
This requires a nominal wind farm of 3MWHr capacity to fill the storage system for off hours ....
Pumped reservoir storage systems are inefficient - assume 70% efficiency for the charge/discharge cycle.
This in turn implies an operational rating for the wind farm of 1MWHr + 2MWHr / .70 ( = 2.8MWHr ) for a grand total of 1MWHr + 2.8MWHr = 3.8MWHr
Let us say 4MWHr grand total, including a bit of margin ....
This brings the energy system design/installed capacity from 1MWHr to 4MWHr     
( OOPS  .-)
As if this were not bad enough it assumes a capacity for 1 day operation only ! Sometimes the wind just refuses to cooperate for several days - requiring a 1MWHr back-up generator ( diesel, NG ... ) still !
So you install a system of 5MWHr total ....
Choose your own factors but = $$$$$$$$$