Optimizing For Electric Miles: One Reader Does The Math

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2012 Toyota Prius Plug-In Hybrid - production model

2012 Toyota Prius Plug-In Hybrid - production model

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It's one of the main points of dispute among electric-car buyers and advocates: What is the "right" number of all-electric miles?

Some say maximal range is needed, e.g. the EPA-rated 73 miles of the Nissan Leaf or the promised 320 miles of the highest-spec 2012 Tesla Model S (that's not an EPA rating, by the way).

Others say the Chevy Volt, with a 35-mile rating, covers the vast majority of situations, since two-thirds of U.S. vehicles cover less than that every day.

Then there's the 2012 Toyota Prius Plug-In, which gives 6 to 15 miles of electric range--and they may not be continuous, since it will switch on its engine under hard acceleration.

Our reader John C. Briggs decided to look at data from the Fuel.ly website to shed some light on the real-world impact of the two approaches in the Volt and the plug-in Prius.

The website lists 12 plug-in Priuses tracked for 23,086 miles, which collectively produced an average 68 mpg. By comparison, the regular 2012 Priuses on the same site got 48.4 mpg. This means 29 percent of the miles traveled by the Prius Plug-Ins are electric miles--better than he would have expected.

In the case of the Volt, the site shows four 2012 Volts traveling 20,188 miles and getting 86.8 mpg, plus two 2011 Volts traveling 13,405 miles and getting 123.3 mpg.

If we assume the Volt gets the EPA-rated 37 mpg when running on gasoline, that translates into 57 and 70 percent of the miles run on electricity respectively. That's a good number, but as good as Briggs had expected.

We would also note that the data doesn't appear to show average trip length or how often each model is recharged . If one car travels longer average distances without plugging in, that will skew the results.

If, for example, plug-in Prius drivers take many shorter trips and plug in for a couple of hours often, their electric percentage may be higher than Volt drivers who do long-distance highway runs. And the reverse is true as well.

2012 Chevrolet Volt

2012 Chevrolet Volt

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(Of course, purists will point out that the Nissan LEAF runs all its miles made on electricity, and rightly so.)

Granted the number of vehicles on fuel.ly are small, and the cars belong to the earliest of early adopters.  However, if the numbers hold up over time, it could show that the while the plug-in Prius is only half as good at racking up electric miles as the Volt, it does so with a battery that is one-quarter the size and (presumably) one quarter the price.

That would be impressive if there were a cost advantage to the consumer, but a 2012 Volt starts at $39,995 and a 2012 Prius Plug-In at $32,760.

"The more I think about it," Briggs says, "the less I care about the technology--whether it's extended-range electric in the Volt versus blended-mode hybrid technology in the plug-in Prius."

The Volt "seems" better and the car has better driving dynamics, but Briggs cares most about percentage of miles traveled on electricity. "If I can get to 75 percent of my miles traveled on electric, I don't care so much how that's achieved."

We agree with Briggs that this seems like a great optimization problem weighing battery cost-per-kilowatt-hour, your local electricity rate, the length(s) of your daily drive(s), and how often you're willing to plug in (and whether you can).

And then, what's the metric you are most comfortable using? Car cost? Overall energy cost (gasoline plus electricity)? Minimizing wells-to-wheels carbon emissions? Or simply the satisfaction of knowing you've used as little gasoline as possible?

Those answers will vary for each individual buyer. Different carmakers (Toyota vs. Chevrolet vs. Nissan) have staked their bets on different answers.

Briggs notes he expected the Prius Plug-Ins to log lower electric mileage than they did, and the Volts to travel more on electricity than they did. Clearly more information about buyers and their travel habits is needed to let shoppers work out which is best for them.

But we're fond of data here, and we appreciate Briggs' efforts at crunching the numbers to provide more discussion points.


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Comments (26)
  1. While I understand the limitations of the method used to calculate for this article (fuel.ly fill-ups), but because of vehicles like the Volt, it is going to take a metric OTHER than fill-ups to answer the question.

    A quick glance at a site like voltstats.net takes ALL miles driven into account, and shows that the average fuel mileage on their subscribed Volts is well between 100-150mpg every month since it's been active.

    And then you have people like myself: I've had my Volt a little over 3 months, have driven over 2,000 miles, but have used less than a gallon of gas. Nowhere near needing to fill up.

    It'll be interesting to see how this trend plays out as more data is gathered.

  2. A larger battery pack always seem great, except you are paying for an asset that will rarely be used. Consider all the resources in the top spec Model S. Perhaps the world is better off if the 85 kwh of battery was be put to use into 20 plug-in Priuses rather than being needlessly carried around (most of the time) in one Model S.

    The Plug-in Priuses would fully use that resource, where as the Model S would seldom use the full pack.

    As a result, the net number of electric miles in the USA would be higher with 20 plug-in Priuses than in one Tesla Model S.

    This is not intended to be a knock against the Model S, which I love. I am just thinking about maximizing limited resources to do the most good.

  3. Excellent point - why carry all of the extra battery around when it will be so under utilized? I chose to go 100% electric for that very reason, I rarely need the "extended range" option. This small study confirms that a 100 mile-range vehicle meets the driving needs of most drivers most of the time.

  4. I wonder if you are really able to manage for life with 100% electric or if you still need to take the occasional trip in a gasoline car.

  5. My real-world experience is 16,000 miles in eight months. I do still use my gas burning about once every two months but won't need to if just one DC fast charger is installed in North San Diego county. I've also made two Vegas runs and have always rented a vehicle for that trip.
    So maybe 1/16th of my driving in the last eight months has been with gasoline.

  6. So that would be about 94% of miles done on electricity, which is just excellent.

    I think for a lot of people, 100% electric is just not going to be possible, even if they own a BEV. So it begs the question, if I can do (pick a number) 80% of miles electrically in a plug-in hybrid/EREV, is that about as good as it gets.

    Granted, there are other significant benefits to BEVs, but most people will still not be able to do 100% of their miles electrically, close maybe, but not 100%.

    It reminds me of solar PV. Sure I can get 100% of electricity from the sun in the summer, but need to use fossil fuels in January.

  7. And also thank you for showing me fuely.com even if I only have another few days to use it!

  8. My experience (two months) is that I only stretched the 100mi range of my Active E once and only had one trip that I used an ICE because I was concerned with the range. My personal opinion is that battery capacity providing more than 100 miles is non-optimized.

  9. Great work John! You're even starting to sound like Better Place's Mike Granoff. He often makes this point when people bring up super huge ranges and super huge batteries for ocassional long trips. You have to cart them around when you're just running down the road for milk.

    In many ways the same is true of the entire machinery needed for range extended cars but aside massive infrastructure investment outside of the cars, the US is going to have these for some time to come.

    It does sound like very few people can live with just a pure EV with now infrastructure for longer journeys. I do know, however, that there are people right now buying Better Place's cars in Israel who will not have this luxury.

  10. Yes, carrying around large batteries has downsides in both physics and economics. Limited range (100 miles) BEV, EREVs, plug-in hybrids, and battery swapping all are attempts to address these issues.

  11. Your point is well taken, but there is one social benefit, even if the Model S range is rarely used, the halo effect of people driving 100 Miles to go ski on battery and then driving back that same day is very positive. That indirect validation and aspirational model encourages more people to buy BEVs.

  12. John B, why didn't you look at Voltstats.net for a better sample size of Volt data? It actually gives you the %EV usage too. For example, last month the average was about 76% EV usage. Second, you really have to compare data from the same months. As you can see on Voltstats, the percentage usage varies a bit, mainly due to the temperature. I am sure the PIP numbers will be lower when driven through the winter.

  13. Thanks for the feedback. Fortunately the 76% is not too much different from the numbers that I saw. But more data is definitely better.

    The reason for using Fuel.ly was that I was trying to get a single resource that had comparable data on multiple vehicles.

    I will be interested to see if the %EV miles drops over time as owners shift out of early adopter mode.

  14. There are, of course, enormous sampling problems involved here - the tiny number of cars and driving habits being sampled, the dissimilarity of trips,the incomplete sampling of seasonal trends (long trips will heavily favor the Prius over the Volt) and so on. Also ignored are long term costs, which are (currently) enormous for an electric car - imagine owning a Tesla Model S or Volt and 8 years down the road being presented with a $40K (or $10K) bill for a new battery pack - or a very depressed price if car is sold. Any analysis that doesn't take into account battery replacement costs is woefully inaccurate. And if EPA figures are accepted for the Volt's gas mileage, then their estimates for the Prius' gas mileage should also be used

  15. With alll due respect, the flaw in your statement regarding long term cost of an electric car is the use of "imagination" as a variable.

  16. There is a lot to digest there.

    I was focusing on the optimization of battery size versus percent of mile traveled by EV.

    Let's say you get a car with a 4KWH battery that the pack costs $2400, that full 4KWH will be used extensively and you will get good value out of that $2400.

    If you add another 4KWH, the pack is now 8KWH and cost $4800. But this incremental increase in battery size will return less value because it will be used less.

    Add two more increments of 4KWH to get to 16KWH and the pack cost is now $9600, and the later increments in size will be even less of an economic benefit because they will not be used as often.

    So while it is easy to feel that the largest possible battery is the best, it may not be economical.

  17. well wouldn't it be nice if you could buy modular battery addons. So that for those who want to buy an extra ten miles of range, it's a more focused cost? Say the person with the 50 mile commute, who wants to go gas free or the person with the 25 mile commute and no charger at work. What's the problem with letting them add on or buy LE or ER models?

  18. Or replace all the complexity of range extenders with infrastructure outside the electric car to quickly switch batteries!

  19. "if EPA figures are accepted for the Volt's gas mileage, then their estimates for the Prius' gas mileage should also be used "

    EPA figure for the Prius Plug in is 6 miles. If you drive normally (unlike a grandma), you won't even get a single mile out of the PIP. However, in Volt you can stay in EV mode for at least 25 miles...

    Kent, you are just a Volt hater. I have seen enough of your biased posts...

  20. @Xiaolong: I disagree that "if you drive normally...you won't even get a single mile" [of electric range] in the Plug-In Prius.

    As you note, it depends on driving style. If you floor it every time away from a stoplight, the engine will kick on.

    If you accelerate gradually, however, the electric motor is capable of moving the car away from rest and propelling it for a handful of miles at speeds within the 0-to-45-mph range. It won't cover *every* driving situation by a long shot, but it will cover *many* usages in regular traffic. If steady gentle acceleration is driving like a grandma, perhaps more people should do it.

    Have you actually spent any time driving a Prius plug-in?

  21. I can only speak for myself' I bought my PiP 2 weeks ago, did my first fillup on Saturday, 7.65 gallons, on 702 miles, with 55KW of electricity used... 91.7mpg. The tank was also not empty, there was about 1.5 gallons or 75 or so HV miles left still.

    I only recently signed up for Fuely, and only have the 1 refill so far, but most of my 14.9 mile commute each way is in EV, especially so if it's all stop and go traffic (which it often is in Boston). Also my weekend driving is also mostly in EV. I will not be surprised if I get a 900+ mile tank soon. The key is recharging whenever you can, and take advantage of the charging opportunitys, even if only on 120V

  22. Well done Mitch. I also live in Boston.

  23. as always, collected data is really just step one of many in determining the "right" thing. but we also have to look at individual need. i know a guy who got a Volt and plugs in every day downtown and at night at home and after 6 months of ownership i am pretty sure he has not filled his tank yet.

    but there is also the initial cost factor as well. Even if i could afford a Tesla, i doubt i would buy one. 85 Kwh is overkill. even if i did take a trip long enough to recharge, it would take hours plus i am saddled with carrying all that dead weight around the other 99.5% of the time where the Leaf's range would be just fine. for me, i have a Prius and a Leaf. its hard to imagine a better combination

  24. David, Well said, and Well done.

  25. A Chevy Volt and an iMEV would rock.

  26. Volt and Pip are both good in their own ways. But I will take a Volt over Prius Plugin b/c it just drives way better...

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