Sorne Hill wind farm

The Sorne Hill wind farm is a wind farm located in Buncrana, Inishowen Donegal Ireland and erected in 2004. The farm is run by Tapbury Management Limited.

Phase 1 of the farm has 16 wind turbines with a 60 metre hub height and 70 metre rotor diameter giving a total capacity of 32 megawatts. Phase 2 of the farm will which is an additional 6.9 MW of wind power for which turbines have been ordered and are due to be installed in the autumn of 2007. This will make Sorne, at 38 MW, one of the largest wind farms in Ireland.

In one of the largest contracts for an energy storage system in the wind power industry, VRB Power Systems Inc. has contracted to supply a 1.5 MW x 8 hour Vanadium redox battery system. This will be driven from phase 2 of the wind farm and will enable surplus electricity to be generated when demand is low, and will improve consistency or supply to the electricity network

http://www.leonardo-energy.org/drupal/node/959

says the cost of the battery system is $6.3 millions.

French 2006 average grid power use is 55GW (peak use was 87GW), so 1.3TWh per average day.

Cost of storing one average day of France electricity use by this system is thus 682 billions USD (37% of 2006 France GDP). Of course we don't have to store all of it in batteries but this gives a reference number :).

http://en.wikipedia.org/wiki/Battery_%28electricity%29

says that sales of batteries amount was 48 billions USD in 2005.

Hydro (pumped or not) is a nice storage too:

http://en.wikipedia.org/wiki/Hydroelectric_energy_storage

Source for french grid power use:

http://www.rte-france.com/

which has 1996 to now electricity grid use at 30 minute sampling available as zipped excel file.