Dear Pablo,

I was excited to hear that Chevy plans to debut an electric car in 2010. But then a discussion with a friend got me thinking: If the electricity used to charge up an electric car is created by burning fossil fuels, is it better to stick with a gas-powered car with good fuel economy?

It’s true that most electric cars get their electricity from the grid, which, in many states, is made up mostly of electricity from coal-fired power plants. In areas where the power comes mostly from hydro, wind or solar, your electric car would be virtually emissions-free. (Any electric-car owner can achieve this by installing a solar carport at home.)

But we can get at your question in a more precise way. The fact is, you can make an apples-to-apples comparison between an electric- and a petroleum-powered car. An electric car doesn’t consume gallons of liquid fuel, so you can’t measure its use of energy in miles per gallon. Similarly, conventional cars don’t run on electricity, so you can’t use miles per kilowatt-hour. So how do you do it?

A few months ago I had the opportunity to visit the R&D facility ofTesla Motors in San Carlos, Calif., and I spoke with people there about this very issue. While their Web site boasts a 256-mpg-equivalent efficiency, they prefer a more geek-friendly metric. I will attempt to translate it into layperson’s terms.

By looking at the energy input into the vehicle, be it electricity or liquid fuel, versus the distance traveled with that energy, you can come up with a measure of efficiency. The energy unit of choice is the megajoule (1 kWh is equal to 3.6 MJ and a gallon of gasoline contains 132 MJ), and the distance measure of choice is the kilometer.