Tesla Motors has finally unveiled their Model S, an all-electric sedan.  This is the long-awaited follow-up to the Tesla Roadster, their souped-up all electric sports car (with the souped-up price tag to match) that made a splash on the red carpet at the 2007 Oscars, and was never heard from again.  A work colleague asked this question:

Can someone help me understand how charging an electric car with power generated from a coal-fired plant is a good thing? I understand that CO2 emissions from gasoline internal combustion engines are less than coal-fired power plant emissions.  I get the part about buying oil from foreign countries, I'm just talking about net-net emissions.

Ignoring for a moment that this person with an engineering degree was unable to do the math themselves, let's take a look.

Here's what we need to know:

• How much carbon is in gasoline?
• How much carbon is in electricity?
• How much gasoline does our baseline comparison car use?
  
• How much electricity does the Tesla S use?

Tesla claims that the S is "2X as efficient as a hybrid." So we're going to use the Toyota Prius hybrid as the basis for our comparison.  The Prius gets 50 mpg, so we'll assume that the S gets 100 mpg.

Oh, wait, the S doesn't use gas. So 100 miles per gallon doesn't make any sense.  Instead, we'll need to use something I'm going to call "mpg equivalent".  We'll find out how much energy is in a gallon of gas, and convert it into kilowatt-hours of electricity. Here's how it works:

(x miles/gallon) X (one gallon / 125,000 Btu [1]) X (3412 Btu/kWh) = (x miles / kWh electricity)

100 mpg equivalent becomes 27.3 miles per kWh. Now we can do our emissions comparison.

Burning one gallon of gasoline produces 20 pounds of carbon dioxide [2]. This is constant no matter where you are.  The amount of carbon produced per kWh of electricity, however, will vary with your geographic region.  The area serving Houston (and most of Texas) averages 1.3 pounds of carbon dioxide per kWh generated.  Check this chart if you want to find emissions in your area.

(pounds of carbon dioxide / mile) = (pounds of carbon dioxide / unit energy) / (miles / unit energy)

For a gasoline car, this is (20 pounds) / (mpg).  For an electric car, this is (1.3 pounds) / (miles per kWh).

A Prius that gets 50 mpg produces 0.4 pounds of carbon dioxide per mile.
The Model S that gets 100 mpg equivalent produces 0.5 pounds of carbon dioxide per mile.

Uh-oh.

Even though the Model S uses half as much energy, it produces 25% more carbon when charged off the grid in Texas. End of story, right?

Well, that does answer my colleague's question, but there is more to the story.  The amount of carbon related to burning gasoline is unchanging, but the carbon related to electricity generation can be reduced all the way to zero. If you get your electricity from a 100% renewable source, either by installing solar panels on your roof or by signing up to get your electricity from a company like Green Mountain Energy, you can reduce the amount of carbon your electric car is responsible for.  The success of the electric car as it pertains to global warming relies on a reduction in the amount of carbon that is released while we generate electricity.

Plus, I'll repeat a point that I've made before: global warming is a symptom.  The problem is sustainability.  A car that is twice as efficient addresses the problem of sustainability, which will inevitably address the problem of global warming.

With all that being said, I don't think the Tesla will succeed; but it has less to do with the carbon problem and more to do with the $50,000 price tag.

Now back to my colleague. This guy has a degree in engineering and was either unable or unwilling to do the math required to answer his question.  This, to me, is a serious problem.  If we as a nation are going to have a meaningful dialogue about the role of energy and the environment in public policy, then we need to be able to understand what it is we are talking about, instead of throwing up our hands in frustration. Or relying on other, less helpful sources.