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 miles per gallon. 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 m.p.g. 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 regional mix of coal-dependent utilities serving Denver diminish the car’s benefits as a global-warming fighter.

The U.C.S. 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 and plug-in hybrids to reduce 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 E.V.’s 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.

The attempt to forge a simple message has created mistaken impressions, too. During a discussion with business leaders and journalists at the French consulate in New York early this month, the chief executive of Nissan and Renault, Carlos Ghosn, doubled down in declaring an environmental edge for electrics over the best gasoline models.