The "burning" question for electric cars is how to compare them with gasoline. Last month I wrote about how wrong the EPA's 99mpg number for the Nissan Leaf was, and I gave the 37mpg number you get from the Dept. of Energy's methodology. More research shows the question is complex and messy.

So messy that the best solution is for electric cars to publish their efficiency in electric terms, which means a number like "watt-hours/mile." The EPA measured the Leaf as about 330 watt-hours/mile (or .33 kwh/mile if you prefer.) For those who really prefer an mpg type number, so that higher is better, you would do miles/kwh.

Then you would get local power companies to publish local "kwh to gallon of gasoline" figures for the particular mix of power plants in that area. This also is not very easy, but it removes the local variation. The DoE or EPA could also come up with a national average kwh/gallon number, and car vendors could use that if they wanted, but frankly that national number is poor enough that most would not want to use it in the above-average states like California. In addition, the number in other countries is much better than in the USA.

The local mix varies a lot. Nationally it's about 50% coal, 20% gas, 20% nuclear and 10% hydro with a smattering of other renewables. In some places, like Utah, New Mexico and many midwestern areas, it is 90% or more coal (which is bad.) In California, there is almost no coal -- it's mostly natural gas, with some nuclear, particularly in the south, and some hydro. In the Pacific Northwest, there is a dominance by hydro and electricity has far fewer emissions. (In TX, IL and NY, you can choose greener electricity providers which seems an obvious choice for the electric-car buyer.)

Understanding the local mix is a start, but there is more complexity. Let's look at some of the different methods, staring with an executive summary for the 330 wh/mile Nissan Leaf and the national average grid:

• Theoretical perfect conversion (EPA method): 99 mpg-e(perfect)
• Heat energy formula (DoE national average): 37 mpg-e(heat)
• Cost of electricity vs. gasoline (untaxed): 75 mpg-e($)
• Pollution, notably PM2.5 particulates: Hard to calculate, could be very poor. Hydrocarbons and CO: very good.
• Greenhouse Gas emissions, g CO2 equivalent: 60 mpg-e(CO2)

Theoretical pure energy conversion

This is approximately what the EPA used. They looked at the inherent chemical energy in a gallon of gasoline -- this is roughly 120,000 BTUs. (It varies based on ethanol content and season.) Only the USA uses BTUs, but by surprising and useful coincidence, a BTU is very close to a kilojoule, and you can use that for back of the envelope calculations if you prefer joules, the metric unit of energy.

Electrical energy is a much lower entropy form of energy, and you can't convert the chemical energy of gasoline or other fossil fuels into electrical energy at 100% efficiency. Not even remotely close. Mainly we generate heat from the chemical energy of the fuels, and turn that heat to electricity. You can refresh your "Carnot Knowledge" for more on the limits of this, but the reality is that our power plants extract about 30 to 50% of the chemical energy, first into mechanical energy (spinning turbines) and then into electricity.

At perfect conversion, you get about 36 kwh in a gallon of gasoline. If you could do that, which you can't.

The EPA was not alone in this. The automotive X-Prize, which rewarded the best 100mpg car faced the same problem of how to compare gasoline and electric cars. It was controversial, and I believe they were in error when they selected the theoretical number, giving a huge advantage to the electric cars in the contest. It was so high that many presumed that it was pointless to do anything but an electric car, but surprisingly the winner, the Edison2, was a gasoline (or rather Ethanol 85%) car.

As you might guess from the name, the Edison2 team started thinking they would be electric. They learned they could do better with gasoline because they could make the car very lightweight. Their car got barely 100mpg, even though the near competitor electrics were rated at 170 mpge (miles per gallon equivalent.) In reality, 100mpg real is better than 170 mpg-e(perfect) and the Edison2 is a deserving winner.

Even the fans of perfect conversion accept there is about a 7% loss in transmission lines, and some further losses in chargers and the battery system, so they don't use the totally perfect number. The EPA used an 8% lower number than perfect, presumably for this reason.

Just using the base energy (heat)

Most of the electricity in the USA is generated by heat. The Department of Energy studies all the power plants in the USA, and their efficiency is calculated and averaged. They include the nuclear plants, which use heat, but generate it from nuclear reactions rather than fossil fuel. People react in many different ways to this methodology on nuclear -- but they are only about 20% of the grid so variations in the methodology there don't affect things a lot.

The DoE method says a kwh at the plug takes 10,300 BTUs, not the 3400 of perfect conversion. It's a very large difference, a factor of almost 3 to 1.

This method is also pure in that it deals with the real energy being released, either by burning gasoline, or coal, or natural gas, or "burning" nuclear fuel. It would be silly, but you could even imagine burning gasoline in power plants (and some power plants do burn oil distillates) to make the comparison more direct.

A fair criticism, however, is "who cares about the heat released?" The heat is waste, except in cases where the power plants are urban and pipe their heat out to provide hot water and interior heating for buildings. In the car it also heats the interior in winter and defrosts the windows.

The cost

One reason to compare gasoline and kwh is to find out the cost of driving. This is a pretty reasonable comparison. At the national average of 11 cents/kwh, the 320 watt-hours/mile of the Leaf is about 3.5 cents/mile. That's pretty good and about 85 miles for the cost of a $3 gallon of gasoline. Electricity is cheaper than gasoline, no doubt. To be fair, gasoline has about 15% tax put on it to support the roads, which electric cars should also pay, and that would bring it down to about 75 mpg(dollar-based). The price of electricity also varies locally. In California, the tier system makes heavy users pay 32 cents/kwh, which would make the electric cars more expensive than gasoline, but I believe that there is a special law which lets owners of electric cars avoid the tier system and pay a more basic rate.

This does not factor in the cost of the battery. The jury is still out on the cost of the battery as an "expendable" part of the car. Some worry the batteries need replacement at a certain rate which makes them cost as much as 10 cents/mile if viewed that way. Others report that vehicles like the Prius are lasting a long time on their batteries. Most agree that batteries do wear out and so eventually they do need a cost/mile associated with them. Of course, internal combustion engines also wear out, as do many other parts of cars.

The pollution

Another way you might want to compare your kwh and your gallons would be by the pollution emitted. Indeed this is probably the main thing in people's minds, since nobody is buying electric cars to save money right now, though they might in the future.

There are, alas, many ways to look at pollution. The biggest one today is greenhouse gas (GHG) emissions, which I will cover below. While we're all about GHG today, the reality is that gasoline and coal emit a lot of pollution of other forms. At present, nuclear power and hydro don't emit much pollution at all, but people are very divided on how green they are. Nuclear waste and the risk of leaks troubles many people, and the destruction of valleys and habitats and fisheries done by hydro is worse than the air pollution of other modes in the minds of different groups.

You have to judge for yourself how you want to compare nuclear and hydro to the other forms. They are about 30% of US power production, and much more of power in Europe and Canada for example.

Gasoline spits out tons of carbon monoxide, as well as nitrogen oxides and various hydrocarbon volatiles compared to coal. Gasoline emits a fair amount of particulates (soot) but diesel is far worse. Coal generated electricity has 2-3x time particulates of gasoline per mile driven and triple the acid-rain causing sulfur dioxide. There are arguments in both directions but I think coal is the big loser here, particularly in the particulates. It is not well known, but particulates are a huge cause of death. California calculated that 9,000 people die per year due to particulate pollution -- that's more than die from car crashes. And this is a state that does not burn much coal!

It is true that many of these deaths are older people whose systems can't handle the damage particulates do to their lungs but the numbers are staggering and trump just about anything here. This makes a strong argument that with coal powered electricity, the mpge(deaths by pollution) is lower for electric cars, though it's hard to figure an exact number. (As a plus for coal, the plants are sometimes located further from people than highways are.)

When I get more research on the amount of PM2.5 emitted per mile by cars and per kwh by power plants, I will update it here. To do this I'll need more data on all the pollutants from the various sources, and also data on how dangerous they are. For example, some of the hydrocarbons are carcinogenic, and we need numbers on the scale of their effects.

Greenhouse Gases

Lots more research is being done on calculating "grams of CO2 (or equivalent) per mile" as a way to compare electricity and gasoline. If global warming is your big issue, this is the one hat you'll focus on. This also is highly locally variable, with very high numbers in the coal states and nice numbers in the hydro and nuclear powered areas.

Research I have seen suggests that coal-powered electricity has about 1000g of CO2 per kwh, while natural gas has about 500g. The others are much lower, though some argue that because of the large amount of energy needed to make today's solar panels, even they incorporate about 150g per kwh.

I've also seen credible arguments that due to the fact that about 1.5% of the natural gas pumped in the USA leaks into the air, and that methane is anywhere from 80 times (20 year average) to 20 times (100 year average) more potent a GHG than CO2 is, the figure for natural gas generation should be higher, more like 700g. As in all total cost "well to wheels" calculations you will find a lot of variation in the final numbers, but I think these are close enough for rough calculations.

The DoE cites 588 grams CO2 per kwh as the national average -- I think the NG leakage number pushes it higher. You will also see well-to-wheels numbers for gasoline around 11.5 kg of CO2 per gallon. From this, we can work out a national average number of 19.5 kwh in a gallon of gasoline, GHG based. That gives the Leaf 60 mpg-e(GHG) on the national average, and something like the same 37mpge we got from heat in a coal state but a much better number (closer to that perfect one) in a nuclear and hydro state. (Yes, it's possible to do better than perfect conversion when it comes to GHG emissions, in fact you can do an almost infinite number if you had completely GHG-free electricity.)

Fuel Content Factor

As a bizarre sign of the politics behind this, read the math in the DoE's electric and hybrid car report done 10 years ago. You'll see in their math that they want to use the heat formula (real energy used) but then they have to multiply it by something called the "Fuel Content Factor." It's amusing to read how they describe it, it's effectively a constant pulled out of thin air because politically they wanted to make alternative fuel vehicles look better. It's huge -- 6.6 -- so it means the DoE's 37mph on the LEAF would get multiplied out to 246 mpge! Problem is, had the EPA used that number, it was more than double the perfect energy conversion number and it would have looked really ridiculous. However, for a while before the cars came out, Nissan did tout numbers in this region, and so have other electric car makers. I do believe electric cars can be better, and particularly on the cleaner grids, but introducing a made up fudge factor for political reasons is a bad idea, because people will stop trusting you when they find out the reality.

So how are electric cars doing?

I'm studying this because as part of my robocar research, I have been predicting that we could gain a lot of environmental and efficiency benefits by switching people to electric vehicles.

What I've learned is that at least for now, the use of traditional sedan sized electric cars not a big win compared to hybrid gasoline cars like the Prius, at least on the average grid. It is a win in the low-coal places like California and the high-hydro and nuke places like Washington state. It's a definite loser in the high-coal places. I refer you to my earlier post and the linked flash application to see details of your own local power grid. I believe the local power companies should all publish these numbers on their web sites and on your power bill.

We can do better, though. We are working on making electrical generation cleaner. We can plug natural gas leaks (it just wasn't worth it from a financial standpoint but is worth it from a GHG standpoint.) We can put in more renewables. We can also use more biodiesel which is lower on particulates and sulfur.

Most of all, we can go for lighter more aerodynamic cars, particularly the single person commuter cars that robocars make palatable to the commuting public. What the X-prize showed was that going electric was not necessarily the answer, but that going lightweight was. And there are also lighter electric cars. The Aptera reports around 100 watt-hours/mile, and small electric velomobiles have gotten down to 30 watt-hours/mile. We are unlikely to get that low in practice, but I believe that 50-60 is a likely goal for single person urban commute vehicles.

Other factors

Electric cars mostly charge at night, which means they tend to use baseload power. What that is varies from place to place. Where there is nuclear, it will tend to that as nuclear plans don't shut down easily. Coal is another baseload provider, unfortunately. Hydro and natural gas can be turned on and off quickly and thus tend to be used for peak load. Wind runs at night more, PV solar entirely in the day.

Fuels for electricity are almost entirely domestic. Significant amounts of oil are imported, sometimes from unfriendly areas.

Emissions from cars are done in the urban areas. Power plant emissions sometimes are out of town. That matters for PM2.5 and Sulfur etc. but not for GHG.

Don't forget that the USA has the dirtiest power grid in the western world nowadays. Drive your electric car in Canada and you can put on a big green smile, pollution and GHG wise. Almost as good in mostly-nuclear France and other countries that have kicked the coal habit. On the other hand it is debatable if they should even licence an electric car in places that have 97% coal power like West Virginia.

What about solar cars?

Quite commonly people will say they just plan to run their electric car on solar and thus have no pollution at all. People go all irrational for this idea -- literally on my block there was a story that got national attention when a neighbour who was trying to have enough solar panels to power his car got a legal order forcing another neighbour to cut down his redwood trees which had over the years grown to shade some of his panels. Leaving out the issue of how much energy is needed to make the panels, there are some serious misconceptions here. I've explained this before but it's worth adding again here.

There are no solar electrons. To properly meet green goals, solar panels must be connected to the power grid. When you connect panels to the grid, you help make the grid a tiny bit greener, and that's good, but you don't make your driving green.

If you don't connect your panels to the grid, it's very wasteful. Panels not connected to the grid instead handle live loads or charge batteries. They might charge the batteries in the car, or they might charge a second bank of batteries which then charge the car. The wasteful latter solution is needed because the car simply isn't at home during the sunny part of the day for most people. It's home at night. Solar panels that just charge a car would throw away most of their power most of the time. But even without that factor, the reality is that batteries only take the full output of panels when the batteries are fairly discharged. When your batteries are only partly discharged, they throw away much of the power of the panels. When your batteries are full -- and most people insist on designing a system that makes sure the batteries fill up, both to give them power they need and to maintain battery life -- the solar power is entirely thrown away.

Throwing away so much of the power of your panels is non-green for several reasons. One is the power it took to make them and the emissions involved with that. The other is that, if you did grid-tie, you could be feeding that power into the grid, and thus reducing demand on grid power plants to burn fuel. A decision to not connect because you want no part of the grid is an environmentally destructive decision. This does not apply, of course, if you have decided to live out in the country beyond the grid. There it was your decision to live there that made the difference.

You may find this circular, but once you agree that you need to be on the grid with your solar power, you now have to accept that your driving is not emissions free, even though your net emissions are very good or even negative. Drive more and you will use more grid electricity from the very slightly greener grid you helped make. Each mile you drive will burn the fuel of the grid, and you can't get away from it. You're being green with your grid-tied panels but your driving is orthogonal to that. If you live in a coal state, you should actually put up solar panels and feed them to the grid, and drive a hybrid rather than an EV or PHEV! That's because the power from your panels will reduce demand for coal and you should put every watt-hour you have into that rather than avoiding gasoline.