In 2012, there were 121 gigawatts of operating natural gas combustion turbines that contributed about 3% of overall electricity generation. The average capacity factors of these combustion turbines varied significantly by time of day and region, as shown in the graph above. Combustion turbines as addressed in this article do not include combined cycle unites that operate at higher efficiency than simple combustion turbines.

via Today in Energy http://www.eia.gov/todayinenergy/detail.cfm?id=13191