This is a really common problem with renewables: you generate most when it's light & windy and the rivers have high flows. You use most when you come back from work. Factories & major industry runs 24*7.
So, how to store the energy from the time when you generate it to the time when you use it?
Not an easy problem to solve - batteries work (but are expensive) up to domestic scale. Pumped water storage only works when you can find a reservoir on a hill. Even geothermal heat storage is tricky.
But some clever German researchers think they may have cracked it - generate standard natural gas (that would power heating boilers etc) from the spare electricity along with water & carbon dioxide.
Sounds clever, but will it come off?
Watch this space...
More at: http://www.fraunhofer.de/en/press/research-news/2010/04/green-electricity-storage-gas.jsp
Cheers
Simon