The Best Cars for the Climate? What a Recent Report on Electric Cars Gets Right — and Where We Need More Analysis

August 13, 2013 | 3:56 pm
David Reichmuth
Senior Engineer, Clean Transportation Program

A new report by Climate Central (CC) compares the emissions from electric vehicles (EVs) to gasoline-only cars and examines how the emissions change depending where you are driving in the country. My interpretation of the results: EVs are less polluting than the average new gasoline vehicle everywhere in the U.S. and EVs are better than the best gasoline car in many parts of the country.

While this is the same conclusion UCS’s own 2012 study, State of Charge, some stories about the CC report (“Jolt to electric cars: Hybrids may be the greener choice”) appear to have the two bodies of research in conflict.

View of Plug-in Prius recharging port

For many Americans, refueling with electricity is a cleaner option. Photo: Toyota UK

There are important differences between the CC study and our own. These differences are mainly in the assumptions made in the study and the interpretation of the results. Our report considered the emissions from driving electric and gasoline vehicles and we did supplemental analysis that considered emissions from manufacturing. The CC report examined both the emissions from building the vehicle and the emission from using the fuel (either electricity or gasoline). The methods used by CC for both determining manufacturing and electricity emissions have drawbacks that have significant consequences on their results.

Despite these differences, the underlying conclusions of this study and our own remain the same: EVs are in fact cleaner than the average gasoline vehicle around the country, they are the cleanest choice in some regions, and EVs will get cleaner as the grid gets cleaner.

Emissions from building electric vehicles are overstated

Three of the methods used in the CC study result in higher electric vehicle manufacturing emissions when compared to gasoline-only vehicles:

  • First, the authors chose to use 20-year global warming potential estimates instead of the more commonly used 100-year measurement. The net effect is a 12 percent penalty to manufacturing emissions, which magnifies the next issue.
  • Second, in estimating vehicle manufacturing emissions, the CC researchers tried to calculate the emissions from building a Prius by assuming that the smaller gasoline engine and electric motors of the Prius would result in less manufacturing emissions. Their simple calculations result in a Prius having less manufacturing emissions than those from a conventional gasoline car, which is a result that I have not seen before from any other analysis. This simplification was taken because the study they relied on did not calculate values for a gasoline-only hybrid. The study that underlies the CC report also is the most pessimistic to date and assumes European energy sources for vehicle production. Fortunately, there is a manufacturing emissions analysis of U.S. vehicles completed in 2012. This study estimates that emissions from building a gasoline hybrid would result in 1,400 lbs. more CO2-equivalent emissions as compared to a conventional vehicle. Incorporating this change would change the CC results in several states, increasing the relative benefits of EVs.
  • Finally, comparing vehicles over their full, useful life is important. Using shorter vehicle life times — such as 50,000 or 100,000 miles as the CC study does — skews the results by overemphasizing manufacturing emissions and ignoring the full effect of fuel savings. New vehicles today are designed to last at least 150,000 miles. Making this change alone in the CC report would make the Nissan Leaf, relying on U.S. average electricity, have lower per-mile emissions (0.64 lbs CO2eq/mile) than every gasoline-only car.

Electricity used in a state isn’t always produced there

Because of the interconnected nature of generation within regional electric grids, power produced in one state can be used in another state in the region. If we look instead at the grid-level data, the electricity emissions will be less extreme. It is understandable why one would want to look at individual states as the CC study did, but this route produces results that don’t match the way power is actually produced. A recent study by Weber et al highlights the difficulty of assigning emissions to political boundaries, rather than the physical layout of the electric distribution grid.

For example, CC reports that electricity in Massachusetts is relatively dirty compared to its neighbors. However, Massachusetts uses more electricity than it generates, so it imports electricity generated elsewhere on the regional grid. The remaining power comes from neighbors on the same grid, states and provinces that have lower emissions. In the last year that data was available (2008), imported power had GHG emissions 59 percent lower (on a per MWh basis) than in-state generation, as Massachusetts imported 27 percent of its electricity from places like New Hampshire and Quebec.

UCS used regional electric grids as the smallest region of analysis in our report for this very reason; they are the smallest area where vehicle emissions can be meaningfully measured with currently available public data. Using state-level data appears to produce more detailed results, but unfortunately that data can only show what the power plants are in a state, not what the emissions effect is of plugging in a car.

Electric vehicles will only get better as electricity gets cleaner

It’s important to consider not only emissions of EVs today, but how they can change in the future as we move toward cleaner sources of electricity. Even if you only look at the 13 states identified as best for battery-only EVs in the CC report, 34 percent of the vehicles in the United States are in those states. Those states also include California and New York, leaders in encouraging adoption of plug-in vehicles.

Their analysis also identifies 28 states where a plug-in vehicle is the lowest-emitting vehicle. Those states have two-thirds of the vehicles in the U.S., meaning even this arguably pessimistic analysis shows that most drivers could reduce emissions with a plug-in vehicle. Because many states that encourage plug-in vehicles also have or are moving to cleaner electricity, these cars will reduce global warming emissions. At the same time, the gasoline in the U.S. is not getting cleaner, especially if dirtier sources like tar sands become more common.  Plug-in vehicles sold today will be on the road for the next decade and longer. The electricity that they use is likely to be cleaner going forward, making the advantage of plug-in vehicles relative to gasoline cars higher over the life of the vehicles.

So what should buyers looking for a less-polluting new car do? My simple advice: Plug-in electric vehicles AND very efficient gasoline vehicles, including hybrids, are all good choices when looking to reduce climate change emissions and reduce oil consumption from our vehicles. That’s something we can all agree on.

About the author

More from David

David Reichmuth's work focuses on analyzing new vehicle technologies and advocating for policies that support the increased electrification of transportation. Dr. Reichmuth has testified at hearings before the US House of Representatives, the California State Legislature, and the California Air Resources Board, and he is an expert on California’s Zero Emission Vehicles regulation.