Amongst the several articles we reviewed that tackle the question: which emits more carbon dioxide, a gas-powered car or an electric vehicle charged from 100% coal power, many leave out an important concept, which is the last key road bump before we bring it all across the finish line. The electricity grid is not 100% efficient.
The kilowatt-hours (kWh, a unit of electricity) that leave the power plant are not the same as the kWh that arrive in your home. Transmission lines lose power. Electrical sub-stations lose power. Low voltage transformers – those grey cylinders that sit on your power line poles – lose power. The grid is good, but by no means 100% efficient. Which means that the carbon intensity of electricity from a coal power plant – which last time we determined was between 2.31 pounds of CO2 per kWh on the low end to 3.38 pounds of CO2 per kWh on the high end – actually should be a little higher.
If the grid is 90% efficient then we take our range – 2.31 to 3.38 – and divide by 90% giving us a new range: 2.57 – 3.76 pounds of CO2 per kWh of electricity that makes it into your home.
And now for the final lap.
• We know the carbon intensity of gasoline per mile: 0.65 pounds of CO2 per mile.
• We know the mileage efficiency of electric vehicles: 2.91 miles per kWh
• We know the carbon intensity of electricity that arrives in your house from 100% coal power plants; 2.57 – 3.76 pounds per kWh.
• All that’s left is to find pounds of CO2 per mile for the coal-powered electric vehicle.
Which is just a matter of dividing the carbon intensity per kWh (2.57 to 3.76) by the mileage efficiency (2.91 miles per kWh) or:
• 2.57 pounds CO2 per kWh / 2.91 miles per kWh = 0.88 pounds of CO2 per mile
• 3.76 pounds CO2 per kWh / 2.91 miles per kWh = 1.29 pounds of CO2 per mile
So the range for a 100% coal powered electric vehicle is 0.88 pounds CO2 per mile to 1.29 pounds of CO2 per mile. Which is greater than the carbon intensity of a gasoline powered vehicle at 0.65 pounds of CO2 per mile. Surprising!
But it’s important to realize that, as with any model, much depends upon the initial assumptions. For one, if we assume a gasoline powered car only gets 20 miles per gallon, then it’s carbon intensity jumps to 0.98 pounds of CO2 mile. Second, our high end coal estimate entailed using the estimate of 8,800 BTU released per pound of coal, but in many cases this number is over 10,000 BTU; the coal number is probably more often closer to the low end estimate (0.88 pounds CO2 per mile) than the high end (1.29 pounds CO2 per mile).
And then there is the reality that there are hardly any regions that have 100% coal power. In California, the carbon grid intensity is about 0.49 pounds of CO2 / kWh using EIA data summarized here.
So assuming 90% grid efficiency, and 2.91 miles per kWh, the carbon intensity of driving an electric car in California is 0.19 pounds CO2 per mile. Almost 4x better than gasoline powered cars.
And even if you charge your vehicle in more coal-heavy Wyoming, where the grid intensity is 2.11 pounds CO2 per kWh, then your electric car is using 0.81 pounds of CO2 per kWh, right at parity with a 25 mpg gasoline car, and easily outpacing a typical sport utility vehicle or truck which get far worse mileage.
And thus far we’ve left out the financial component as well. Which is very exciting for electric car enthusiasts:
• If gasoline costs $3.50/gallon, then a 30 mpg gallon gasoline powered car pays 11.7 cents per mile, and a 20 mpg gas car pay 17.5 cents per mile.
• If electricity costs $0.10/kWh, which is the national average and closer to what electricity costs at night even in expensive areas, and a car goes 2.91 miles per kWh, then it costs 3.4 cents per mile to drive an electric car. Less than a third the price of driving a gasoline car.
• Plus, while gasoline prices are extremely volatile, nighttime electricity costs should remain relatively low and flat. It is daytime and peak energy costs that are the real driver of electricity price hikes.
We hope this series was educational. We certainly learned something. The impressive inefficiencies and carbon intensities of coal power plants leave even more to be desired than we thought before researching this issue.
But fortunately we already live in world where electric cars are no more carbon intensive than gasoline guzzling ones and usually much better. We live in a world where energy costs are on the side of electric vehicles and where battery research is speeding along (stay tuned for a post on this shortly). And most importantly, we live in world where there is a real way forward away from foreign oil dependence, away from distributed particulate and SOx and NOx pollution, away from the destabilizing geopolitics of oil, and away from energy intensive and destructive process of ripping up the earth for materials only to combust them into the atmosphere and start all over again. That is why we, and many others, are so excited about electric cars.