Editor’s Note: EarthTechling, always looking to bring you interesting cleantech reading, is proud to repost this analysis courtesy of new partner Do The Math. Author credit goes to Tom Murphy, an associate professor of physics at the University of California, San Diego.
A typical efficient car in the U.S. market gets about 40 MPG (miles per gallon) running on gasoline. A hybrid car like the Prius typically gets 50–55 MPG. In a previous post, we looked at the physics that determines these numbers. As we see more and more plug-in hybrid or pure electric cars on the market, how do we characterize their mileage performance in comparison to gasoline cars? Do they get 100 MPG? Can they get to 200? What does it even mean to speak of MPG, when the “G” stands for gallons and a purely electric car does not ingest gallons?
This post addresses these questions.
Using the Wrong Measure
Okay, first of all, MPG (called fuel economy) has always been a poor choice of units. We don’t usually put a gallon of gas in the car and drive as far as it might take us. Rather, we tend to have a destination in mind and care about how much gas it will take to get there. The inverse, GPM (called fuel consumption), would therefore be a better measure, and is akin to the measure used in some parts of the world (e.g., Europe’s liters per 100 km).
Besides this philosophical advantage, the GPM approach has a numerical advantage. If you have an old truck that gets 12 MPG and car that gets 30 MPG (let’s say both travel comparable distances in a year) and you want to replace one of the vehicles, are you better off replacing the truck with a 16 MPG model or the car with a 40 MPG model? Framed this way, the car looks like a better choice: a gain of 10 MPG vs. 4 MPG.
Let’s say for numerical simplicity that you want to go 240 miles in both. In your initial situation, it takes 20 gallons for the truck and 8 gallons for the car, for 28 gallons total. If you replace the car, the same journey will take 6 gallons for the car for a total of 26 gallons. If you replace the truck, the new one takes only 15 gallons for a total of 23 gallons. It’s amuch better deal to replace the truck, even though the MPG gain seemed less impressive. We would have had to replace the 30 MPG car with one that gets 80 MPG to achieve the same 23 gallon result. If we had been using GPM instead of MPG, we’d never get confused on this point, and would therefore make smarter decisions. Try adding the values of 1/MPG to see that this is true, as in the following table.
|Scenario||truck MPG||car MPG||gal/240 mi||truck GPM||car GPM||total GPM|
A New Measure for All Cars: Electric and Gasoline
With this lesson in mind, we would like a measure of energy per distance traveled. I’m a big fan of picking a universal energy unit and applying it to all forms of energy we use. Among the zoo of energy units: Joule, kilocalorie, Btu, Therm, kilowatt-hour, gallon of gas, barrel of oil, etc., I prefer the kilowatt-hour as a common standard. I like this because my favorite unit is the Watt (is it wrong to have a favorite?), and using the kWh makes it straightforward to flip between energy and power. Like ya do.
So I’ll bow to both the American and the scientist in me and try a unit as schizophrenic as an American scientist must be when it comes to units: kWh/mi. Lovely.
Let’s look at our 40 MPG sedan. A gallon of gasoline contains 36.6 kWh of heat energy when combusted, in this case taking us 40 miles down the road in the process. So this car uses 0.915 kWh per mile.
We tend not to be fond of puny fractions: especially in America where we like our numbers BIG. So let’s take a hint from the Europeans and use kWh/100-mi. Now our sedan has an energy consumption of 91.5 kWh/100-mi. Using this measure, we desire a smaller number for our car. Energy consumption in these units for gasoline-driven cars can be calculated as 3660/MPG kWh/100-mi. A 12 MPG Hummer has an energy consumption of 305 kWh/100-mi, while a Prius, at 50 MPG has an energy consumption of about 73 kWh/100-mi.
How do electric cars or other electric/hybrids stack up? In order of performance: the Chevy Volt gets 35 miles from a 16 kWh battery for a consumption of 45 kWh/100-mi (see note below); the Nissan Leaf gets 73 miles from its 24 kWh battery for 33 kWh/100-mi; and the pricey Tesla has a 244 mile range using a 53 kWh battery, for 22 kWh/100-mi. The MPG equivalent of these three figures is approximately 80, 110, and 170, respectively. All are much better deals than gasoline cars deliver, primarily because the electrical drive system is far more efficient than the typical 20% gasoline engine.
[Note: a reader informs me that his Volt uses 30 kWh/100-mi, the difference being that the battery is only allowed to use 10.4 kWh of its 16 kWh capacity.]
If you pay $0.10/kWh for electricity, these three cars travel 100 mi for costs of $4.50, $3.30, $2.20, respectively (triple this for Hawaii). At $3.50 per gallon of gasoline, a car getting 50 MPG will cost $7.00 to travel the same distance. So in almost all cases, the “fuel” cost is less to the consumer at current prices.