“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Tuesday, July 30, 2013

A quick note on the eGallon

Screen shot of my results from DOE eGallon site
In a previous post I mentioned the eGallon concept from a Department of Energy (DOE) web site. It purports to tell a visitor how much he or she would pay to drive as far in an electric vehicle as a gallon of gas takes them in an "average vehicle." It breaks down only as far as by state (or U.S. average). So, for example, if I use California, it tells me that a gallon of regular gasoline costs $3.99 and that my eGallon costs $1.53.

But my average mileage over the life of my vehicle is 50.86 m.p.g. At my most recent fill up I paid $4.059/gallon. I'll use the Nissan Leaf for a comparison, the vehicles are broadly similar in important ways. Each has a Cd (drag coefficient) of 0.29 and, while the frontal area of the CT200h is a bit larger, the Leaf weighs more. The Leaf is rated by the EPA to consume 29 kWh/100 miles for the 2013 model year.

So, on a gallon of fuel, I go 50.86 miles. The Leaf would need (50.86/100)*29 kWh = 14.75 kWh to go that distance. If I assume that the charging system is 85% efficient, I'd pay for 14.75/.85=17.35 kWh. On my most recent electric bill I paid $0.1611/kWh for electricity above the "basic lifeline" rate, so these 17.35 kWh would cost me $2.80 and that's the price of my eGallon. Quite a difference between that number and $1.53, the "true" number is 83% higher whereas the number for my gasoline cost is not far away from what I actually pay. The computed eGallon price would be even further from ReGallon cost ("Rob's eGallon") if DOE had used the 2013 model year numbers for the Leaf in lieu of previous years' 34kWh/100 miles. If I use the 2013 model year number for the Leaf and the EPA combined estimate (42 m.p.g.) for the Lexus CT200h that I drive, an eGallon would cost $2.31, only 51% higher than the site's number.

You can read about their methodology here. The confounding factors are the actual cost of electricity and the fuel economy utilized for the ICE (internal combustion engine) vehicle. For reference, the plot below (you can click it to enlarge and be able to read the numbers) shows an AeGallon ("actual eGallon") for a range of actual fuel economies from 12 m.p.g. (the driver currently in a vehicle getting less than that is not a likely candidate for an EV) to 70 m.p.g. (a hypermiler in a Prius). For this plot, I'll use the same electricity consumption as the DOE site uses, i.e., 35 kWh/100 miles, a blended rate from 5 top selling EVs. Electricity prices on the plot range from $0.09 to $0.20 per kWh. You can calculate your number yourself, it's as simple as 0.4118*(m.p.g.)*(electricity cost per kWh). You'll note that, for combinations of high mileage vehicles and expensive electricity, the eGallon may be more expensive than a gGallon (i.e., a gallon of gasoline).

On the plot, the "front" axis is m.p.g. for the vehicle being replaced with an EV, the rearward extending axis is the price of a kilowatt hour of electricity, and the vertical axis is the price of an eGallon in dollars. You can see that, for low mileage vehicles being replaced, the eGallon is quite inexpensive, regardless of electricity costs. But as replaced vehicle fuel economy climbs, the eGallon becomes much more expensive. The DOE site simply uses a single fleet average fuel economy (28.2 m.p.g.) and does not correct for the 85% charging efficiency I estimated.

5 comments:

Arezoo said...

I was checking Lexus CT 200h specs; it's 42 mpg. So, I'm confused where does 50.86 mpg come from? Is that based on your experience? I'm thinking to get a Ford C-Max. They claim 45mpg. We'll see if they lower the number again.

Rob Ryan said...

Good morning Arezoo. Yes, the 50.86 m.p.g. is the ten tank-full moving average of my mileage. I use so-called "hypermiling" techniques to significantly exceed EPA m.p.g. estimates.

I have exhaustive records going back eight years and three vehicles on every gallon of gas I've used, along with analysis of the data from a statistical and technical viewpoint.

I'm kind of obsessive!

Rob Ryan said...

Correction: 50.86 is the m.p.g. over the vehicle's lifetime (a bit over two years). The ten tank moving average is currently 52.47 m.p.g.

Arezoo said...

That's a lot of data! Impressive! Do you use all hypermiling techniques? I'm particularly curious about the maximum speed. For me, it takes a lot of effort not to exceed the speed limit!

Rob Ryan said...

I use almost all of them. I drive 55 m.p.h. on cruise control in the right lane. If I'm going down a hill I may shut cruise control off and coast to a higher speed, though now that I'm in a hybrid that captures some of that downhill energy to charge the battery, I don't go as fast as I used to down hills. In my previous car, I'd get going 85 or 90 and had the engine shut off on very long steep hills. In the hybrid, the computer shuts the engine off and uses some of the potential energy being converted going down a hill to electrical energy through the generator and subsequently into chemical potential energy in the battery, to be used when needed.

Anyway, yes, I do drive slowly. It was hard at first (back in 2005 when I started) because my habit then was to push the accelerator to the floor on startup and go as fast as traffic would allow (and sometimes faster). But I got terrible gas mileage!