Nissan Leaf EPA rating of "99mpg" is, sadly, a lie.
Nissan is touting that the EPA gave the new Leaf a mileage rating of 99mpg "gasoline equivalent". What is not said in some stories (though Nissan admits it in the press release) is that this is based on the EPA rating a gallon of gasoline as equivalent to 33.7 kwh, and the EPA judging that the car only goes 73 miles on its 24kwh battery.
There is a huge problem with these numbers. If it were possible to convert perfectly, a gallon of gasoline actually has about 36kwh, so possibly the EPA is factoring in the 7% loss of electrical distribution. But in reality it isn't even remotely possible to convert fuel to electricity perfectly.
I have written and update on comparing gasoline and electricity with more details.
The Department of Energy, for example, offers a number which puts under 13kwh as the energy equivalent of a gallon of gasoline. That's how many kwh you get out of the plug if you burn coal, gas or oil with roughly the same energy as that gallon of gas. With the DoE's number, the Leaf is getting a combined mileage of around 36 mpg-equivalent. That's not a bad number, but there are many gasoline cars that do better than that. Even a Lexus hybrid does similar to that. This is no minor error, it's a massive one, and it's highly unlikely that Nissan or the EPA are unaware of it. This gives the impression of an attempt to make the Leaf seem way, way better than it is to promote electric cars. The problem with that is that when people learn the truth, they are going to be unhappy, and will be soured on electric cars, Nissan and the EPA.
Now I will agree that there is justifiable debate over the right way to do this calculation. The DoE works from its calculation of the average efficiency of power plants in the USA. People in areas with more efficient power will do better using electricity than those close to old coal plants (which are the big drag-down here.) The DoE also counts BTUs in nuclear plants (which provide about 20% of U.S. energy) as BTUs even though no fossil fuel is burned and no greenhouse gas is emitted. People must judge for themselves how "dirty" they think nuclear BTUs are, and how to value an electric car in areas where most of the electricity is nuclear. Even harder to judge are the 10% of US kwh that come from hydro. Hydro doesn't even have BTUs or pollution, though it does come with environmental destruction. If you live in the Pacific Northwest or parts of Canada where most of the power is from hydro, you may judge the 99mpg number as more realistic, though in this case the concept of a gasoline equivalent is stretched pretty thin.
If you live in California, which burns almost no coal and gets most of its power from natural gas, and then nuclear, the real number isn't as bad as the national average, but it's still nowhere close to 99mpg. If you live in a place that is almost all-coal, like Utah or New Mexico, electric cars are not so great an idea -- their only environmental advantage is that the fuel source is domestic rather than imported, and the coal is burned elsewhere, not right next to you.
There are other electric cars that are more efficient than the Leaf, but the big reality is that to really beat out the 50mph gasoline hybrids you need to make your car lighter.
"But wait," some people say. We can run our electric car on solar or renewables and all is wonderful! Don't get me started on this. There are no solar electrons. Installing renewable generation can be a good idea, but you must tie it to the grid for it to work. Not tying solar or wind or other sources to the grid is highly wasteful, because the power is discarded any time the battery is not empty (or worse, not connected.) Grid tie makes the grid greener, and people who do that can feel good about it if they do it well, but it does't make driving more than a tiny smidgen of a percent greener than it was.
Shame on Nissan and the EPA. I hope that at least, Nissan will only sell the car in places with electricity that is well above average in quality, and refuse to sell it in places where the power is mostly from coal.
Not that I don't understand the motivation. Had the EPA rated the car with the DoE methodology number of 36mpg, it might well have killed the car at the starting gate. It's an interesting moral question if it's right to lie to kickstart a technology which will become better with time. They could also have lobbied for a more reasonable but generous mpg, perhaps derived from the best natural gas plants, which would have offered a number in the 50s. Not nearly as exciting but not a car-killer, though the comparison to the Prius or Insight would not look so good.
It would have been best if they had just developed a new standard, like watt-hours/mile or miles/kwh, and leave it to the press and local power utilities to publish local conversions between "kwh" and gallons. (Not the dealers, they can't be trusted of course.) It actually would be quite handy if every power utility were to publish, for each zone the local efficiency of the power grid in terms of BTU/kwh or greenhouse effect/kwh.
Update on Chevy Volt: The numbers for the Volt were released. As a plug-in Hybrid that can go 35 miles on its batteries and then has a gasoline engine, they rated it as 97mpg while on the battery (similar false number to the Leaf) and 37mpg while on gasoline. These numbers are actually roughly the same when using electricity at the grid national average.
Sad to say, but if you live in a place where the power comes from coal, the math seems to say you should remove most of the batteries and save the weight.