Doublecheck My Math: Hybrids Can Be Greener Than Electric Cars

By · November 05, 2010

Calculator

Photo by Afroswede via flickr/creativecommons.

It started with an assignment from Home Power magazine. They asked me to create a graph to visualize the relative efficiency and green-ness of various cars—so I decided to undertake a comparison of CO2 emissions and cost for a typical gas, hybrid and electric car.

I specifically wanted to know about the relative carbon emissions of pure electric cars in different parts of the country, depending on the coal/renewable mix at various electricity generation plants. People repeatedly ask, “Are electric cars really green?” and I want to finally shoot down EV critics—as we frequently do on PluginCars.com—with some hard data.

So, I reached out to experts in the field of environmental lifecycle analysis, like Costa Samaras at the non-profit Rand Corporation, who led me to the EPA’s eGrid analysis, which provides a count on the kg of CO2 equivalents for a kWh of electricity in different regions of the country.

The latest data comes from 2005. Is it accurate? Is the methodology correct? Has the grid gotten a lot greener since 2005? I don’t know, but it’s the ONLY data out there with CO2 counts for different regions. On top of those questions, Costa’s research makes it very clear that it’s impossible to trace the exact source of electricity for any individual household. We have a national grid, and electrons don’t respect state or regional borders.

Break Out the Calculator

Stick with me folks, the numbers are going to start flying.

With kg/CO2 per kWh numbers in hand, I was able to convert that number to pounds of CO2 per mile by using a factor of 3.6 miles per kWh of EV driving. That’s lower than the rule-of-thumb for EV efficiency—4 kWh per mile—and way lower than many EV drivers experience, but that’s what the EPA used to account for total consumption, including losses during transmission and charging, for their 2012 to 2016 fuel efficiency rules. (If you don't like it, don't call me. Call Lisa Jackson.)

I took a similar lifecycle approach to determining the pounds of CO2 for a 30-mpg gas-powered car and a 50-mpg hybrid. Yes, I know that most Toyota Prius drivers, for example, don’t routinely get 50 miles to the gallon, but that’s just one of many assumptions that I decided to make. (Careful hybrid drivers do get 50-mpg, and I anticipate a lot more will in coming years as hybrids, along with all cars, get more efficient to meet rising federal standards.)

I used 23.7 pounds of CO2 for each gallon of gas burned. That comes right from the DOE Argonne National Lab’s GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation) model, that includes the primary energy source extraction, transportation and processing for gasoline. Is that accurate and complete? Who knows?

So, if you look at a driver who clocks 15,000 miles per year, then it’s easy enough to calculate CO2 for 500 gallons (for the 30-mpg car); 300 gallons (for the 50-mpg hybrid); and 4,167 kilowatt hours for the pure EV. (15,000 miles divided by 3.6 miles per kWh produced the 4,167 number.) For
fuel costs
and electricity rates, I pulled up regional numbers from the DOE’s U.S. Energy Information Agency. I couldn’t find some of the regional gas prices, so I found those from gas-buddy-type sites on the web.

Sum It Up

With all this data, and all those assumptions, I started crunching numbers for 10 different cities. The cities are just representative locations, because again it’s impossible to know where the electrons came from.

For San Francisco, near where I live, I used a CO2 average of three different grids (from eGrid)—listed in kg/CO2 per kWh of 0.1 to 0.183 as .36, .45 and .66 (this is right now)—to indicate emissions for 15,000 miles of driving as follows:

  • 4,345 pounds of CO2 for the EV
  • That’s tremendously cleaner than the 30-mpg car’s 11,850 pounds of CO2
  • It’s also way better than the 50-mpg hybrid’s 7,110 pounds of CO2

On a cost basis, the EV also comes out way ahead. Even at $0.1552 per kWh—I know that time-of-use rates might be significantly lower, but that’s what the USEIA is using for California—the EV beats the competition. I used $3.12 for the cost of gas to reveal the cost of a year’s worth of driving in San Francisco:

  • The EV tallied at $647, compared to the hybrid’s $936, and the gas car’s $1,560.

So far, so good. But the numbers become troubling when you start looking at parts of the country with dirty grids and expensive electricity. Warning: Don’t shoot me. I’m just the number cruncher and I’ve told you about all my assumptions. But for Phoenix and Boston, I show the EV emitting at least a couple of hundred pounds MORE CO2 respectively than the 50-mpg hybrid. And in Boston, the annual cost of driving the EV only beats the hybrid by $191.

Postscript

I've probably confused everybody with my back-and-forth calculations, and the incorrect descriptions, but to me, the situation is confirmed (as much as it can be confirmed based on government numbers). In a couple of cities where electricity is dirty, the 50-mpg hybrid actually beats the electric car for low CO2. At first, that surprised me, but I quickly came to see it as the exception that proves the bigger and more important rule: for a myriad of reasons—from less local air pollution to greater reduction of our dependence on foreign oil and lower fuel costs—the pure electric car is as green as it gets. At the same time, the critical importance of the conventional (no-plug) hybrid—especially considering that adoption rates (sales) that are likely to be faster and higher than EVs—should be appreciated. I'm sure users will fill in a bunch of other gaps in the comments below.

The final graphic showing a comparison of 10 different cities will appear in Home Power in a couple of months. I'll let you know when it hits the stands.

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