Electric buses have lower overall carbon emissions than diesel or natural gas buses, even in parts of the country where the electricity grid is powered by fossil fuels, according to a new Union of Concerned Scientists (UCS) study. The conclusion echoes the UCS' findings on electric cars.

Like electric cars, electric buses produce no "tailpipe" carbon emissions. But a common argument against electric vehicles is that charging them form power grids that use fossil fuels to generate electricity still makes them pretty dirty. That isn't the case, the UCS study argues.

The average diesel bus operating in the United States gets 4.8 mpg, but it would need to achieve 12 mpg to have the same lifecycle carbon emissions as an electric bus, according to the study. And while burning diesel will never get cleaner, the lifecycle emissions of electric buses can be reduced by eliminating fossil fuels from the grid.

Electric buses have the lowest overall emissions in the Pacific Northwest, which relies heavily on hydro-electric power, California, which has large wind and solar installations. The Carolinas, Mid-Atlantic states, New York, and New England also showed lower emissions, although nuclear power is a bigger part of the energy mix in these states. Nuclear power plants do not generate carbon emissions, but remain controversial because of concerns over accidents and disposal of radioactive waste.

In the states with the cleanest grids, electric buses had an even clearer advantage to their diesel counterparts. A diesel bus would have to achieve 37.3 mpg, a respectable figure for a new passenger car, to equal the emissions of an electric bus in upstate New York, according to the study. It would have to achieve 21.2 mpg in California and 19.8 mpg in New England to equal the emissions of electric buses running off the local power grids.

Electric commercial vehicles don't get as much attention as electric passenger cars, but they will be an important part of the emissions-reduction puzzle. Heavy-duty vehicles like buses only make up 5 percent of vehicles on the road today, but they account for 25 percent of transportation-related emissions, according to the UCS.

Nationwide, 85 transit agencies have begun electrifying their bus fleets, a UCS statement said. Los Angeles is aiming for an all-electric bus fleet by 2030, with the first 25 electric buses expected to be delivered in 2019. Seattle's King County Metro wants to make its bus fleet all-electric by 2040. Other agencies, including New York City's Metropolitan Transportation Authority, have purchased smaller numbers of buses, or conducted trials.