The percentages reflect the assessment of the capacity value of wind at the time of peak electric demand (often late afternoon in the summertime). (eia.gov)
With warmer summer weather and increased electric demand for air conditioning, demand will increase, requiring increased output from both coal - and natural gas - fired generators. (eia.gov)
Also, total electric demand will grow more rapidly as dwindling oil supplies cause a big shift toward electrically powered equipment of all kinds. (futurepundit.com)