The traditional metric for automobile efficiency is miles per gallon (MPG) of gasoline used. But what does MPG even mean for a vehicle that doesn’t burn liquid fuel? Most of us would agree that the Leaf certainly does use fuel — after all, something must create the electricity, and in the US that is generally some sort of fossil fuel. So theoretically we should be able to create an MPG equivalent, or MPGe for short, to measure the fossil fuel use of electric vehicles.So, what's wrong with that?
For more than a decade, within the EPA and the Department of Energy (DOE), a number of different approaches to this problem have been discussed. With the release of the numbers for the Nissan Leaf, we now know what approach the EPA is taking, and results are depressing for those of us who would like to see transparency and adherence to science in the Administration. This Administration has a huge intellectual investment in electric vehicles and financial investment in the Chevy Volt. Like a shady company trying to pump up their stock by choosing a series of highly questionable accounting conventions, the EPA has chosen an approach that grossly overestimates the MPGe of electric vehicles.
The Nissan Leaf was rated at 99 MPGe. To reach this number, the EPA created a conversion factor between a quantity of electric energy, measured in kilowatt-hours (KwH) and a volume of gasoline, measured in gallons. They did this be dividing the potential energy or heating value of a gallon of gasoline (115,000 BTUs) by the energy in a KwH of electricity (3412 BTUs) to get a conversion factor of 33.7 gallons per KwH. Using this factor, they can convert miles per KwH of electricity in an electric vehicle to an MPGe that is supposedly comparable to more traditional vehicles.
The single biggest energy loss in fossil fuel combustion is the step when we try to capture useful mechanical work (ie spinning a driveshaft in a car or a generator in a power plant) from the heat of the fuel’s combustion. Even the most efficient processes tend to capture only half of the potential energy of the fuel. There can be other losses in the conversion and distribution chain, but this is by far the largest.Damn that Second Law of Thermodyanamics... The EPA accounting method doesn't account for the energy "wasted" in the conversion of fossil fuel to the energy used to power the car. In a gasoline engine that conversion takes place on the car itself, while in a Volt, that conversion takes place in a power plant miles from car. Out of sight, out of mind, out of the calculation.
So how big is that difference? If it's small, maybe it's not worth mentioning.
DOE looked at the electrical generation efficiency, and determined that only 32.8% of the potential energy in the fossil fuel becomes electric energy in the average US power plant, which it further reduced to 30.3% to account for transmission losses. However, they realized it was unfair to charge electric vehicles for these losses without also charging gasoline-powered vehicles for the energy cost of refining and gasoline distribution. They calculated these as adding 20% to the energy it takes to run a gas-powered car, but rather than reducing existing MPG standards by this amount, they instead gave a credit back to electric vehicles. The 30.3% electric production and distribution factor was increased to a final adjustment factor of 36.5%. This means that the conversion factor discussed above of 33.7 gallons/KwH must be multiplied by 36.5% to get a true apples to apples MPGe figure. The end result is startling.Good, but not great. Modern hybrids do much better than that using gasoline. A friend of mine recently reported 62 mpg for a Prius on a long trip.
Using the DOE’s apples to apples methodology, the MPGe of the Nissan Leaf is not 99 but 36!
In some areas, particularly the Pacific Northwest, a majority of electrical power is produced by hydropower. In others, nuclear or wind energy may produce significant fractions of the power. In these cases, the calculations work out more favorably.