Whenever Scientific American posts an article about electric cars, we see comments along the lines of "electricity comes from burning fossil fuels, so electric cars don't really reduce your carbon footprint, and the electricity is more expensive than gas anyway," possibly with more expletives. Ignoring issues of civility, it's a good question. There's no such thing as a free lunch: the energy to run a car has to come from somewhere and cost something.

My parents got a Chevy Volt last October. The Volt is a plug-in hybrid. After being charged, it can run for 25 to 50 miles on electricity, depending on temperature, terrain and driving style. Then a small gasoline engine kicks in, either charging the battery or powering the motor directly, depending on what is most efficient for driving conditions. In light of the concerns raised in recent electric car comments sections, I thought it would be interesting to figure out my parents' costs and emissions for running the Volt. This is just a case study, and I promise that my parents are not a representative sample of the country. But they are real people who are using a Volt for everyday driving, so their experience might help elucidate the environmental and monetary costs of driving a plug-in hybrid.

First, I should say that my parents love their Volt. It feels good to drive, and my dad likes being an early adopter of new technology. He enjoys all the ways that their vehicle keeps track of its energy usage, and as a statistics geek, he likes running the numbers himself to see what his mileage is. They didn't choose the Volt because they thought it would cost less or reduce their carbon emissions; my dad was in the market for something sporty, and my mom talked him out of a Corvette and into a Volt. For her, one of the big reasons was energy independence. Even if it comes from dirty coal, electricity is not foreign oil.

The Volt takes about 10 hours to charge using a regular 120 volt power outlet, but my parents got a 240 volt charging station in February as part of the EV Project. The EV Project paid most of the cost of installation of the charging station, and they are collecting driving and charging data from thousands of plug-in vehicles in order to make recommendations about charging infrastructure as electric vehicles get more common.

I logged on to the various websites where my parents' information is recorded to see how much electricity they've used and what kind of mileage they're getting. There were 11 months of the following data: total miles driven, electric miles, gas miles and electricity consumption.

As of Wednesday, when I looked up the information, my parents had traveled 10,102 total miles in their Volt, 9,186 (90.9 percent) by electricity and 916 (9.1 percent) by gas. They had used 2,437 kilowatt-hours of electricity and 24.4 gallons of gas. They can usually get about 43 miles on a full battery; my dad's office was 19.5 miles away from home, so he could usually get to work and back on one charge.

For individuals, the cost per mile is an important measurement. The car came with a full tank of gas, so my parents have only bought 18.9 gallons of gas for a total of $66.70 (my mom jots down the amount of gas and price whenever she refuels). For the other 5.5 gallons, we'll use $3.50 per gallon as an estimate for the typical premium gas price for the area in October 2011, giving us another $19.25 of gas costs for a total of $85.95. They have a 12-month fixed rate energy plan. Their rate has changed a little bit in the time that they have had the Volt, but on the more expensive plan, they paid $0.0885 for each additional kWh. Using that price, we get $215.67 spent on electricity. That brings us to a grand total of $301.62 for the 10,102 miles they drove, or three cents per mile. For comparison, with an estimate of $3.50/gallon gas, a 2012 Prius that gets 50 miles per gallon costs seven cents per mile, and my 2004 Saturn station wagon costs 14 cents per mile on a good day.

Of course, many Volt owners buy them for environmental reasons, so knowing the carbon footprint is important, too. My parents, who live in Dallas, have a 100 percent wind power plan, so in some sense their carbon emissions from electric driving have been negligible. But it's not like there's a power line from a wind turbine to their house. They're sucking energy out of a big pool, so it's reasonable to look at the way electricity is generated in Texas to get an idea of the environmental impact. (Even more difficult to suss out is the "marginal" energy source, or what will produce the extra electricity used by the Volt. Scientific American took a look at this problem back in 2010. That study analyzed what the energy sources would be for a future with a much larger number of electric vehicles on the road. For my purposes, the current sources of Texas electricity seem more applicable.)

According to the U.S. Energy Information Administration, energy in Texas is 45.4 percent natural gas, 36.5 percent coal, 10 percent nuclear and 7 percent renewable, with small contributions from other sources. Using the EPA estimates on carbon emission, this energy mix means that every kilowatt-hour of electricity generates 1.336 pounds of carbon dioxide. For my parents' electricity usage so far, that's 3,256 pounds of CO2. The EPA estimates that each gallon of gas releases about 19.59 pounds of carbon dioxide, so the Volt has also emitted 478 pounds of carbon dioxide from gasoline for a total carbon footprint of 3,734 pounds of carbon dioxide. If they had driven the 10,102 miles using only the 37.5 mpg gas engine, the Volt would have released about 5277 pounds of carbon dioxide. A Prius would have generated 3,958 pounds of carbon dioxide over the same number of miles.

Unfortunately, the carbon emissions of the Volt are not limited to the gas and electricity used to drive the vehicle. Producing the vehicle and its monster battery pack take a lot of energy. Several groups are looking at lifecycle emissions for all types of vehicles, but in this post I am just trying to crunch some numbers myself, and lifecycle emissions involve too many unknowns. (If you're curious, here are two conflicting reports.) Lifecycle emissions might make a fuel-efficient gasoline car a better environmental choice now, but one point that many proponents of electric vehicles make is that it may be easier to implement more efficient technologies at one power plant than in every internal-combustion engine on the road.

With their typical driving, my parents pay less per mile than they would in a traditional hybrid or gas-powered car. They also emit less carbon dioxide. Overall, they are happy with the car, and we had a lot of fun running the numbers together. Thanks, Mom and Dad!