Nissan LeafHow do the Leaf’s emissions stack up?Finally. If you don’t like being dependent on oil — but find that you do need to drive — you’ve got at least one decent option. The Nissan Leaf is the first mass-produced, mass-market electric vehicle to hit the U.S. sales floors in … well, essentially forever. (Yeah, I know about the Tesla and the EV1. But the former is too expensive to be in the range of most families, and the latter was never really offered for sale — you could only lease it.)

The Leaf’s a bit pricey, but for many families there are federal tax breaks that can help. And while the car has hit a few bumps in the road — including a recent recall to reprogram the starter, and some complaints about shorter than expected range — the car’s overall reviews have been pretty positive.

But if your main concern is the climate impact of your driving habits, how does the Leaf fare? The EPA label says that the car gets the energy equivalent of 99 miles per gallon — 106 mpg in the city, 92 mpg on the highway. Pretty good, in other words!

But the EPA also says that the car emits “0” pounds of climate-warming emissions each year. And while this is technically true, it’s also misleading. No, the Leaf doesn’t have a tailpipe spewing carbon-laden exhaust. But the electricity the car runs on doesn’t magically appear out of nowhere. And even in the Northwest, blessed as we are with lots of hydropower, some of the electricity that comes out of our sockets started out as coal or natural gas. So despite what the EPA label suggests, the Leaf does have some climate impact.

How much? Let’s run some numbers to see …

There are probably as many ways of calculating carbon footprints as there are eggheads. So I’ll do my best to explain my estimates, without making things too terribly complicated.

The Northwest Power and Conservation Council says that generating the average kilowatt-hour of electricity (enough to power 10 100-watt bulbs for one hour) in the Northwest produces a little over a half pound of CO2. (See the chart on page eight [PDF].)

But there’s a difference between the average carbon emissions and marginal carbon emissions. The marginal emissions represent what power source gets turned up or down when electricity demand goes up or down just a little bit. The Northwest Power and Conservation Council (NWPCC) estimates that in the Northwest, emissions from marginal electricity average about 0.72 pounds per kilowatt-hour. That’s higher than the region’s average emissions, because natural gas generation tends to be “on the margin” (see page 11 [PDF]).

Of course, not all of the power that gets produced at a generating plant even makes it to your home. Some is dissipated as heat during long-distance transmission, or as it’s distributed from the substation to your home. Based on the latest data from the Energy Information Administration (from this page, access the “Table 10″ spreadsheet for each Northwest state), transmission and distribution losses in the Northwest total about 6 percent.

I thought it would be worthwhile to compare the Leaf with a few examples of gasoline-powered cars. I’ve estimated in the past that the lifecycle emissions from gasoline total 25.77 pounds of CO2 equivalents per gallon, including both CO2 from the vehicle tailpipe as well as the “upstream” and trace emissions from fuel extraction, refining, distribution, and combustion.

So based on all this, how do the Leaf’s emissions stack up? Take a look:

Leaf emissions chart

As you can see, the Leaf’s greenhouse-gas performance depends crucially on where its electricity comes from.

Personally, I think that the “right” way to think about the Leaf in the Northwest is the bar in the green: the marginal carbon emissions. And on that measure, the Leaf is a lower-carbon vehicle than the Prius. In fact — and purely coincidentally — the Leaf’s emissions work out to be just about the same as its EPA rating: the equivalent of a gas-powered car that gets about 99 miles per gallon.

But on the other hand, if the electricity that powers the Leaf comes from a coal-fired power plant, then the battery-powered car performs far worse than a Prius. In fact, by these calculations, it’s the equivalent of a car that gets about 30 mpg: better than the average car, but certainly not much to brag about.

I take two things away from all this. First, electric cars in the Northwest appear to be a pretty good deal for the climate. But second — and more importantly — coal-fired power negates all of the climate benefits of electric cars. If power companies in the West were to build lots of coal plants in order to power a fleet of electric vehicles, electric car buyers will be doing the climate no favors. People might as well buy a small-and-efficient car that burns plain old gasoline.

So if you really want to drive green, you ought to focus on getting rid of coal, as fast as possible. What car you drive is important; but where your electricity comes from can make an even bigger impact on the climate.

And a technical note: the NWPCC’s estimates of emissions for gas turbines and coal plants may be a bit low, since they may not include all upstream emissions. That said, their estimates are roughly in line with other life cycle estimates for emissions from electricity generation. Life cycle analysis for natural gas power seems to have particularly wide variation — so take these figures as reasonable estimates, not the gospel truth.