How Green is Your Ev?

Lynne Kiesling

On Monday the Union of Concerned Scientists released an analysis estimating the MPG equivalence of electric vehicles. The point of the analysis is this: taking as given an objective of greenhouse gas emission reduction, how do electric vehicles compare to internal combustion vehicles in that dimension? To do such an analysis requires comparing the GHG emissions across the two types of engines, taking into account that the electricity generation fuel mix varies across the country. Here’s how they did that:

Most drivers are familiar with the concept of miles per gallon (mpg), the number of miles a car can travel on a gallon of gasoline. The greater the mpg, the less fuel burned and the lower your global warming emissions. But how can such consumption be figured for electric vehicles, which don’t use gasoline? One way is by determining how many miles per gallon a gasoline-powered vehicle would need to achieve in order to match the global warming emissions of an EV.

The first step in this process is to evaluate the global warming emissions that would result at the power plant from charging a vehicle with a specific amount of electricity. Then we convert this estimate into a gasoline mile-per-gallon equivalent—designated mpgghg, where ghg stands for greenhouse gases. If an electric vehicle has an  mpgghg value equal to the mpg of a gasoline-powered vehicle, both vehicles will emit the same amounts of global warming pollutants for every mile they travel.

For example, if you were to charge a typical midsize electric vehicle using electricity generated by coal-fired power plants, that vehicle would have an  mpgghg of 30. In other words, the global warming emissions from driving that electric vehicle would be equivalent to the emissions from operating a gasoline vehicle with 30 mpg fuel economy over the same distance (Table 1.1).3 Under this equivalency, the cleaner an electricity
generation source, the higher the mpgghg . When charging an EV from resources such as wind or solar, the mpg equivalent is in the hundreds (or thousands) because these resources produce very little global warming emissions when generating electricity.

This map, from a New York Times feature on the report, summarizes the results:

The results reflect the regional variety in electricity generation fuel mix — hydro power in the Pacific Northwest increases the mpgghg there, as does the predominance of nuclear around Chicago. The results suggest that even in the coal-intensive Midwest and plains states, electric vehicles using coal-generated electricity outperform the standard 4-door 27 MPG sedan in the greenhouse gas dimension.

I found this analysis useful and informative. Frankly, I often take UCS analyses with a grain of salt, because they are an advocacy group and generally start their analyses with presumptions of catastrophic global warming that directs their conclusions, while I think it’s more scientific to make assumptions that weaken your conclusion so that you don’t bias your analysis toward your desired conclusion. This analysis, while still a piece of advocacy, presents the calculations and mpgghg comparisons in a more dispassionate fashion that I found informative. The New York Times also had an article on Sunday summarizing the report.

2 thoughts on “How Green is Your Ev?

  1. I view this well-intentioned study as seriously flawed. You cannot equate a state’s or region’s annual fuel mix with its marginal fuel. Let’s not kid ourselves: In most of the country for most of the year, the marginal fuels are gas and coal. Transmission and other operating constraints make it otherwise only in specific locations and times. Of course, the fossil-fuel consumption may occur at some distance from the state imposing the load, and whether the marginal fuel is gas or coal depends on the relative prices of the two and their penetrations in the regional power system.

    Off-peak load is generally a good thing, though, which you see reflected in off-peak prices in the market systems. Off-peak load generally helps relieve those operating constraints, which have only been exacerbated in areas where off-peak wind generation has been added. But off-peak load will consume wind generation only when it would have otherwise been curtailed, hydro generation only when it would have otherwise been spilled, and nuclear generation only when it would have otherwise been turned down off-peak (or otherwise curtailed). The Pacific Northwest is the one location I can think of where these occur simultaneously and chronically, but only load added within the constrained region can reduce the curtailments.

    The longer-term picture is different, of course, but we are not likely to see more hydro and more nuclear units in response to off-peak load, and we are less likely to see the constrained conditions that place them on the margin today. Energy-based RPS can tie greater off-peak load to greater renewable energy generation, but it’s not clear where that will lead in terms of costs, constraints, and round-the-clock operations of the electric system. A carbon price would raise the cost of coal generation relative to gas, but would ironically place the coal generation that remained on the margin more often than it is now.

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