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> 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.

This would then change the calculation. Wikipedia (http://en.wikipedia.org/wiki/Fossil-fuel_power_station) says that the efficiency for a coal powered generator is only 33%, as opposed to 56-60% of a combined cycle gas powered generator. Based on the above data, if one assumes only hydro carbons are used, for power production, that would give 37% natural gas and 63% coal. Then, the average efficiency is 33.63+60.37= 43%. So, the over all efficiency would be 1.14*43/60=.817 km/MJ, which now doesn't look that impressive. Also, I am not sure if the calculation accounts for transportation loss involved in getting the coal and oil to the power plant.

Given that the next contender, the Prius, has three times the CO2 emissions of the Roadster, I think theres plenty wiggle room for the energy source here.

I understand there has been a sharp rise in natural gas (and a corresponding drop in coal and nuclear) in the past two years, since fracking took off. (I wouldn't be surprised if coal is still at the top though.)

If we could effectively get the oil out of ground in CA (total four hundred billion barrels, that's half of oil in all of Saudi Arabia), it may completely change the game. http://money.cnn.com/2013/01/14/news/economy/california-oil-...

Yes natural gas overtook coal as the leading source of electricity in the USA. The transition was in 2012.

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