It's a lot like one of those math problems that gave you fits in sixth grade: A salesman leaves home in Denver and drives his electric car to a meeting in Boulder. At the same time, a physicist driving the same model electric car sets out from her loft in Los Angeles, heading to an appointment near Anaheim.

For both, the traffic is light, and the cars consume an identical amount of battery power while traveling the same number of miles. Being purely electric, they emit zero tailpipe pollutants during their trips.

The test question: Are their carbon footprints also equal?

The answer may be a surprise. According to a report that the Union of Concerned Scientists plans to release Monday, there would be a considerable difference in the amount of greenhouse gases -- primarily carbon dioxide -- that result from charging the cars' battery packs. By trapping heat, greenhouse gases contribute to climate change.

'State of Charge'

The advocacy group's report, titled "State of Charge: Electric Vehicles' Global Warming Emissions and Fuel Cost Savings Across the United States," uses the electric power requirements of the Nissan Leaf as a basis for comparison. The Leaf, on sale in the United States for more than a year, sets a logical baseline.

The California part of the story is upbeat: A hypothetical Los Angeles Leaf would be accountable for the release of an admirably low level of greenhouse gases into the atmosphere, about the same as a gasoline car getting 79 mpg. But the Denver car would cause as large a load of greenhouse gases to enter the atmosphere as some versions of the gasoline-powered Mazda 3, a compact sedan rated at 33 mpg in combined city and highway driving by the Environmental Protection Agency.

In simple terms, the effect of electric vehicles on the amount of greenhouse gases released into the environment can span a wide range, varying with the source of the electricity that charges them. California's clean power makes the Leaf a hero; the coal-dependent utilities serving Denver diminish the car's benefits as a global-warming fighter.

Source of power

The UCS report, which takes into account the full cycle of energy production, often called a well-to-wheels analysis, demonstrates that in areas where the electric utility relies on natural gas, nuclear, hydroelectric or renewable sources to power its generators, the potential for electric cars to reduce the amount of carbon dioxide emissions is great. But where generators are powered by burning a high percentage of coal, electric cars may not be even as good as the latest gasoline models -- and far short of the thriftiest hybrids.

With gasoline hovering around $4 a gallon and mass-production EVs like battery-powered versions of the Ford Focus and Honda Fit (as well as plug-in hybrids like the Chevrolet Volt, Toyota Prius PHV and Ford Fusion Energi models) either on sale now or coming soon, the report arrives at an ideal time. Its analysis can help shoppers make informed decisions.

It also fills a gap: Many of the existing studies on electric-car efficiency were completed before models like the Leaf came to market; others have expressed their results in science-lab terms like pounds of carbon dioxide emissions per year, not especially useful to consumers. Automakers have not always helped their customers understand the issues, either, typically painting electrics and hybrids with a green brush and an idealistic setting.