The value of existing and potential electricity supply resources depends mostly on how much and when they operate. Because the electric industry currently lacks large-scale storage, supply is adjusted in real time to match demand. Electricity demand varies significantly from moment to moment, hour to hour, day to day, and season to season. Thus, the pattern of electricity demand dictates the type, size, and timing of supply needs.

via Today in Energy http://www.eia.gov/todayinenergy/detail.cfm?id=12711