> However, let’s assume for the moment that the electricity is generated from a hydrocarbon source like natural gas, the most popular fuel for new US power plants in recent years.

Above statement is mostly true in state of California where natural gas generates one third of its total power (source: http://energyalmanac.ca.gov/electricity/total_system_power.h...)

Not so true national wide. According to US Energy Administration, the energy sources and percent share of total for electricity generation in 2011. Note the the combined renewable energy sources is below 10% still in 2011.

• Coal 42%

• Natural Gas 25%

• Nuclear 19%

• Hydropower 8%

• Other Renewable 5%

• Biomass 1.38%

• Geothermal 0.41%

• Solar 0.04%

• Wind 2.92%

• Petroleum 1%

• Other Gases < 1%

(source: http://www.eia.gov/tools/faqs/faq.cfm?id=427&t=3)

Coal is still the king.