‘One of the hidden costs of solar and wind power — and a problem the state is not yet prepared to meet — is that wind and solar energy must be backed up by other sources, typically gas-fired generators. As more solar and wind energy generators come online, fulfilling a legal mandate to produce one-third of California’s electricity by 2020, the demand will rise for more backup power from fossil fuel plants.’
via Renewable energy increase will require use of more fossil fuels – latimes.com.