MPG Comparison: 2012 Volt vs. 2010 Prius

By · February 23, 2012

I suppose I’ve always been a bit of a car guy. At the age of 16, my brother and I already had three cars and performed a nearly successful engine rebuild on our 1957 Chevy convertible. During my adult life as a parent, engineer, runner, amateur cook, and car aficionado, I’ve had several GTOs, BMWs, and other non-performance cars.

However, my 2012 Volt is unique among all of them. Although not as peppy as the 135i that I traded-in on my Volt, the Chevy is almost as much fun to drive.

My recent fascination with electrics started a few years ago when I started a consulting gig for a battery supplier. Although not responsible for the design of vehicles, my job brought me close enough to get bitten by the electric car bug. I knew I had to have one. At this point I must give some credit to my dear wife of nearly 40 years. As an environmentalist of long-standing she convinced me back in 2004 that the new model Prius would be a wise investment. So, now on our second Prius (a 2010 model IV), we have some street credibility when it comes to environmentally friendly vehicles.

Volt and Prius

2012 Chevy Volt, and 2010 Prius in the background.

So How Do They Compare?

I’m an engineer, so you knew there would be some tech talk. To compare my 2012 Volt and 2010 Prius, I want to know which car is better to drive on a trip. For example, if we are planning a 100-mile journey, which car is most economical? How about a 200-mile trip or longer?

Before I could get down to comparing the two vehicles, I needed to install one piece of monitoring equipment for the Volt, i.e. an energy meter that would tell me how much energy my Volt consumes during a recharge. This was necessary because the Volt’s internal readout only tells how much electrical energy (KWh) is used from the battery to travel a given number of miles, i.e. net energy used. I call this the battery specific energy (KWhb/mile).

However, for a valid comparison what is needed is the total amount of energy to charge the battery, i.e. gross energy used. I call this the true specific energy (KWhT/mile). The battery and true specific energies are different because of the pumping and fluid heating/cooling energy required to maintain the battery temperature during charging. The true specific energy should always be greater than or equal to the battery specific energy.

Image 01

This graph shows the gross and net specific energy history over the past three months or so. Again, the difference between these is that the net specific energy is determined using the Volt instrument readout while the gross specific energy is determined using the KWh meter readout. There appears to be a 0.032 KWh/mile overhead associated with the pumps and heaters when charging the Volt’s batteries.

Volt vs. Prius Comparison (mpg$ & mpge) at $3.60 per gallon gasoline. The graph below makes the decision easy. It compares the Volt and Prius based on the energy used (mpge) as well as on the cost of the energy (mpg$). It shows the breakeven trip at around 90 miles or 60 miles depending on whether energy or current cost is the basis of the comparison, respectively.

Image 01

This graph shows the effect of $4 per gallon gasoline on the comparison. This is a very real and I feel reasonable projection given recent trends. The breakeven trip at around 90 miles or 67 miles depending on whether energy or cost is the basis of the comparison, respectively. The two curves overlap at $6.00 per gallon gasoline. This suggests that the true cost of gasoline based on its energy content should be around $6.00 per gallon.

Image 01

This graph shows the breakeven trip at around 90 miles or 67 miles depending on whether energy or cost is the basis of the comparison, respectively. The two curves overlap at $6.00 per gallon gasoline. This suggests that the true cost of gasoline based on its energy content should be around $6.00 per gallon.

Assumptions

Image 01

  1. average EPA gasoline mileage
  2. based on data from my volt between 11/18/11 and 2/16/12
  3. recent price of 89 octane gasoline in my locality
  4. based on recent utility electric bills
  5. average Prius mileage between 9/18/10 and 2/17/12 ~ 50.1 mpg
  6. data averaged from several websites

Conclusions

  • The criterion upon which a comparison can be made between the performance of the Volt and Prius is technically straightforward. If we use as a criterion the amount of energy consumed, the Volt shows a miles driven breakeven of about 90 miles. Whereas, if we use the cost of the energy consumed, then the miles driven breakeven falls to about 60 miles.
  • A clear implication here is that as gas prices rise relative to the electric rate, the $-breakeven will approach the energy-breakeven at around $6.00 per gallon.
  • Of course the electric rate and gas price are both relative to the local market in which the comparison is made.
  • Also, the time-of-year and temperature are additional variables that will affect the comparisons.
  • During the time of my comparison when I had the KWh meter installed there was not much variation in the amount of energy required for pumping and heating the battery. The average remained fairly constant at 1 KWh.

Comments

· · 2 years ago

Very nicely done. This is exactly what someone should look at when considering purchasing a Volt and when deciding when to use it compared to other things.

Ideally, people buying a Volt should drive less than 35 miles per day on a typical day to avoid gas altogether. But on those days where that is not possible your philosophy gives Volt drivers a good guide.

Since you used a Prius this sort of comparision would have longer miles in the Volt's favor for other second cars. And the average price of electricity in the US is around 12 cents/kWhr compared to your 17 cents so people need to take that into account if cost is the issue for them.

· dwl (not verified) · 2 years ago

Interesting read. I didn't see it explicitely stated. But I thought it was worth mentioning that these numbers should be taken in the context of an average daily drive over the course of a year. Rather than in the context of a single trip. Ie, if you average 80 miles per day, you're probably better off buying a Prius right now. If you average 40 miles a day but take a couple 500 mile trips every year, the Volt's still the overall winner. As least if these graphs are the only deciding factor.

· · 2 years ago

The problem with Volt is utility - otherwise I'd be buying one. I want my PHEVs to be larger ... looks like only Mitsubishi Outlander PHEV really meets my criteria.

· Chris C. (not verified) · 2 years ago

Excellent report, Ray, thank you!

I see three minor technical errors in your analysis and one fundamental “error”.

Minor error #1: You are using 89 octane gas, but GM recommends premium gas for the Volt. I can see getting away with 89 if you burn it pretty quickly, but you really should stick with the manufacturers recommendation for the purpose of this calculation (like you did with the EPA MPG number).

Minor error #2: I wouldn’t attribute the gross vs. net kWh difference to just the pumps and heaters. The charger (AC/DC rectifier) efficiency and battery efficiency itself will contribute. You should generalize that definition a bit more. Just my opinion :)

Minor error #3: That electricity cost is highly variable across the US. I am paying $0.055 per kWh, about a nickel per kWh, here in Atlanta. That’s going to push the crossover distance way out. You should run the calcs to show how much it changes for cheaper electricity rates.

Major “error”: I find it very frustrating that people compare the Volt and the Prius on efficiency alone. The Volt BLOWS AWAY the Prius on performance (e.g. acceleration). A Volt has as much torque as a Mustang! On the basis of “fun to drive” the cars are in completely different segments. Plus there are the aesthetic issues, such as quality of interior and general sporty appearance of the car. I really think ANY analysis that compares the two cars should make these points loud and clear — the Volt is certainly more fun to drive than a Prius and STILL beats it on efficiency in most typical driving scenarios. As Dan Akerson says, I wouldn’t be caught dead in a Prius!

Up to nearly 3000 miles on one tank in my Volt,

Chris C., PE

· · 2 years ago

Interesting comparison. Especially appreciate the explanation on the battery specific energy and gross energy as I have been noting that in the data we are collecting on our Volt. Thanks for sharing!

· Nik (not verified) · 2 years ago

Interesting analysis...would appreciate if you can clarify how the mpg$ figures are calculated.

· Chris T. (not verified) · 2 years ago

At $.17/kWh, you should look into putting solar PV on your roof. I'm doing it here and over a 20 year time span (based on warranty; I actually expect the system to last at least 30 years, and maybe more than 50) the per-kWh price is lower than that. Of course I live in an area with a lot of sun (about 5.5 "PSH" per day).

· Edward Ellyatt (not verified) · 2 years ago

A 277 mile trip with expense free opportunity charging yielded 67.5 MPG with Volt #1506. So you see it depends on weather you have to pay for your opportunity charge or not. I have never seen a Prius make a 1 day trip and average 67.5 MPG and you cannot discount opportunity charging. Take Care, TED

· Edward Ellyatt (not verified) · 2 years ago

I also pay $.10 per KWH fpr my e;ectricity in Florida. FP&L does not have a policy to encourage the use of electric cars. Take Care, TED

· · 2 years ago

Thank you, Ray, for these wonderful graphs! As an engineer myself, I appreciate what you are accomplishing here. There is, however, one more line that I believe you should draw on your graphs: gallons of gasoline consumed. I think it is great that you are looking to minimize your energy consumption, but consider also where that energy comes from. Electricity is almost always domestically produced, whereas incremental petroleum is always imported (a gallon not burned is a gallon not imported). Another valid goal is to minimize gasoline, even if you use a little more (electrical) energy to do so. This line will intersect with the Prius' even farther out, well over 100 miles:

Using EPA stickers:
Volt = 37MPG, 35 miles electric
Prius = 50MPG

Going x miles:
Volt uses (x-35)/37 gallons
Prius uses x/50 gallons

To break even:
(x-35)/37 = x/50
x = 134.6 miles

· · 2 years ago

@Brian Schwerdt,

That's a good breakdown of the calculation. Individual results may very and this doesn't take into cost of electricity or premium gas the Volt uses. But it does show unless your a high mileage driver the Volt will use less fuel in most normal circumstances.

· Matt Griscom (not verified) · 2 years ago

Here's an important option you left out: buy a Nissan Leaf, and never buy gas at all! I've owned a 2011 Nissan Leaf for 9 months now, have over 9800 miles on it, and I love it. NO, despite that nasty GM Volt ad, I've never been stranded. It works great for my commute and I've only ever charged it in my garage.

· George Parrott (not verified) · 2 years ago

I would second other comments above about the "cost of electricity." Our home has a 5.5kW solar PV system and that totally WIPES OUT all of our home electricity use cost AND fully covers, using TOU rates and late night charging of TWO EVS (We have both the Leaf and the Volt).

Further, since, at least for us, our daily Volt driving is local, we virtually never see the ICE activated, but our family commuting trips of 200-300 miles are almost all ICE (except for the first 35-45 miles EV range) and at freeway ICE performance. We are getting consistently 40-42.5 mpg under ICE operation.

So I would opine that your review gives a "worst case" scenario for Volt operating costs in terms of the mpg base you impose and the electric rate you further assume, and the neglect of the possibility that the Volt can get that electricity from a solar PV based home networked system.

· · 2 years ago

It will be interesting to compare the Prius Plugin to the Volt, I think. If you drive short hops with an hour or more between, then quick (partial) recharges are possible. And I hope we see many more plugins -- all hybrids should be plugin, in my opinion; and bump up the battery capacity!

Neil

· David Noland (not verified) · 2 years ago

Ray, have you used your energy meter to determine how much power the Volt draws when it's plugged in but fully charged? I have a 220-v charger and get fully charged in about three hours, and usually spend 2-5 hours each day out driving or parked unplugged, so my Volt typically spends 15-20 hours per day in the fully charged/plugged in mode.

Presumably the power draw under these circumstances would vary greatly with temperature, but I was wondering if anybody has any numbers on this. Your 1 kWh figure implies about a 10 percent charging inefficiency (full charge about 10 kWh), but if the car is not driven for a couple of days, the charge-maintaining losses could build up and perhaps be more significant.

· · 2 years ago

This is exactly the analysis that I stated 9 weeks ago in the story http://www.plugincars.com/most-important-plug-hybrid-metric-mpg-when-bat... where I stated:

"The metric that I would like to see on the label is the MPGe at different ranges (maybe in a chart form) that considers the electric MPGe for the AER [all electric range] portion and calculates in the CS [charge sustain] MPG for the remainder. For example, the Volt at 93 MPGe electric, 37 MPG CS and 35 miles AER would show 93 MPGe at 10 miles, 93 at 20, 93 at 30, 78 at 40, 64 at 50, 57 at 60, 50 at 80, 47 at 100, 43 at 150, 41 at 200 and 40 at 300.

By comparing the charts for the different vehicles, you can find the cross over points for the various PHEVs to find out what vehicle best meets your driving style."

· · 2 years ago

maybe someone can create an online calculator that uses the allows you to input a gas price, electric price, MPGe electric, MPG CS and miles AER for different vehicles and it will give you these plots.

· · 2 years ago

Oil Friday afternoon (2/24/2012): $109.77 per barrel.

· Anonymous (not verified) · 2 years ago

Both vehicles are a great alternative to the high price you'd have to pay if you didn't have a hybrid.

HYPERMILING:

http://arizonataylor.hubpages.com/hub/Save-Gas-Cut-Fuel-Costs-Expert-Mon...

· triffle (not verified) · 2 years ago

I was very interested in this article. Surely if the research was made more public, or could be independently verified, the mystery would disappear.!!!!...
Price Comparison

· Gabriel (not verified) · 2 years ago

i used the Distance calculator to check the free truck rate
and it gave me the exact rate details.so you guys can check it..

· john (not verified) · 1 year ago

i used the Distance calculator to check the free truck rate
and it gave me the exact rate details.so you guys can check it..

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