Calculating MPGe

[georgek]

"You mean the "miles from regen" number? This number provides an estimate of how many "extra" miles of range were added purely due to energy captured from regeneration (and stored in the battery)."

WattsUp
Yes, thanks for that . I have wondered about that as well.

 

[dmen]

Nice explanations, Wattsup.

Simplest way to explain where 33.7 kWh comes from- it is the amount of energy contained in 1 gallon of gasoline, according to the EPA. Gasoline-burning cars tend to waste a lot of that energy, both through not capturing all the energy released in burning, and in not being able to recapture kinetic energy when slowing. Efficient gas engines try to address the first part, hybrid braking systems try to address the second part.

Most accurate way to calculate your actual vehicle's MPGe- use a device that measures how much energy is going out to your EVSE. Then simply use the odometer to see how many miles you drive, and how many kWh you use to replace the energy it took to drive that far. Dividing your vehicle's displayed kWh used by 0.8 will give you a ballpark, but actual charging efficiency isn't always 80%. It will be less efficient charging a very empty or an almost full battery, and will be less efficient if you are using plug energy to manage battery temp or precondition the car. So if you do those things, and want a more realistic idea of your MPGe, use a lower charging efficiency like 75% in your calculation.

 

[WattsUp]

Thanks dmen.

Simplest way to explain where 33.7 kWh comes from- it is the amount of energy contained in 1 gallon of gasoline, according to the EPA.

Correct. The part where people really get confused when computing their MPGe is not realizing that charging their car consumes, on average, 20% of that 33.7 kWh (wasted as heat) and that the EPA effectively counts that consumed energy in their MPGe rating for EVs (simply due to their testing methodology of "charge to full then drive to empty"). That typical "charging overhead" of 20% is truly part of the energy required to operate an EV.

So, you either have to divide the car's reported Wh/mile by 0.8 (which is that way I normally think about it, since it yields the total energy used per mile) or multiply 33.7 kWh/gallon by 0.8 (to discount the charging overhead) before performing the division for MPGe. Either way will account for the charging efficiency and produce the correct value.

Here is the EPA's estimated MPGe for the FFE computed using both methods:

Method 1: 33.7 kWh/gallon / (256 Wh/mile / 0.8) = 105 MPGe
Method 2: (33.7 kWh/gallon * 0.8) / 256 Wh/mile = 105 MPGe

The ratio of 0.8 is simply moved around within the equation.

Dividing your vehicle's displayed kWh used by 0.8 will give you a ballpark, but actual charging efficiency isn't always 80%.

That's true. As always, "your mileage my vary". The EPA reports that the average EV charging efficiency is 81% but, you are correct, your own personal charging efficiency depends a lot factors (the battery SOC, the ambient temperature, charging voltage, etc.). 80% is just a good rule of thumb, but whatever your charging efficiency, it cannot be ignored when calculating MPGe.

 

[DavidJohnson]

I got my info from a quick google search that ended in a wikipedia article linked below. Looking back, I see I used the farmula for California instead of the EPA's formula. I'll inclusde the text of the section "Electric and plug-in hybrid electric vehicles" below to help understand how they get their numbers.


http://en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent


This begins the quoted material from wikpedia......
Between 2008 and 2010 several major automakers began commercializing battery electric vehicles (BEVs), which are powered exclusively on electricity, and plug-in hybrid electric vehicles (PHEVs), which use electricity together with a liquid fuel stored in an on-board fuel tank, usually gasoline, but it might be also powered by diesel, ethanol, or flex-fuel engines.

For battery electric vehicles, the U.S. Environmental Protection Agencys formula to calculate the battery-to wheel MPGe is based on energy standards established by the U.S. Department of Energy in 2000:[1][7][8]

MPGe = Eg / (Em * Ee) = 33,705 / Em

where:
a) MPGe is expressed as miles per gallon gasoline equivalent (as shown in the Monroney label)
b) Eg is energy content per gallon of gasoline = 115,000 BTUs/gallon, as set by U.S. DoE and reported by the Alternative Fuel Data Center.[8]
c) Em is wall-to-wheel electrical energy consumed per mile (Wh/mi) as measured through EPA's five standard drive cycle tests for electric cars and SAE test procedures[7][27]
d) Ee is energy per KWatt-hour of electricity (BTU/Wh) = 3.412[8]
The formula employed by the EPA for calculating their rated MPGe does not account for any fuel or energy consumed upstream such as the generation and transmission of electrical power, or well-to-wheel life cycle, as EPA's comparison with internal combustion vehicles is made on a MPGe = 32,600 / Ave (displayed in wh/mi)
MPGe = 32,600 / 316
MPGe = 103MPGe = 32,600 / Ave (displayed in wh/mi)
MPGe = 32,600 / 316
MPGe = 103.

The California Air Resources Board uses a different dynamometer testing than EPA, and considers reformulated gasoline sold in that state. For CARB estimates the formula becomes:[7]

MPGe = 32,600 / Em
This ends the quoted material from wikipedia.....

With all that being said, my calculation should have been:
MPGe = 33,705 / Ave (displayed in wh/mi)
MPGe = 33,705 / 316
MPGe = 106

The only difference between WattsUp (whom I respect tremendously!) and myself is that he is including the inefficiency of charging the battery and the EPA is calculating theirs with what's actually in the battery, hence their "battery-to wheel" description. I stand by these calcs, and in my humble opinion, this should be how we, as a group, perform the calculations. It's how the EPA did it, it's how most of the numbers you'll see published in the media are calculated, and anything else is just contributing to confusion. Just my two cents

 

[WattsUp]

David, thanks for the all the quoted info and kind words.

But, I am afraid you've missed the important ramification of this clause you quoted...

c) Em is wall-to-wheel electrical energy consumed per mile (Wh/mi)

This key point has been the main thrust of my posting about MPGe.

Em absolutely takes charging efficiency into account since the EPA's testing methodology is expressly wall to wheel. They care nothing about, and do not measure directly, the internal battery-to-wheel performance of any given EV. The only thing the EPA does is externally measure the energy from the wall required to fully charge the EV and then externally measure how far it can drive. From these two external observations alone, they compute the wall-to-wheel Wh/mile figure to be used as Em. They never look at the Wh/mile displayed by the car, or look at the odometer, or use anything else reported by the vehicle itself.

So, back to your example. When your FFE reports 316 Wh/mile, that is not Em. That just happens to be your internal "battery-to-wheel" energy usage. It's good to know, but it is not your external wall-to-wheel energy usage. But, assuming typical charging efficiency, we can deduce that the 316 Wh/mile reported by your FFE represents, on average, an Em of 395 Wh/mile, and thus an MPGe of 86.

I really do think I am correct here.

 

[jmueller065]

I agree with WattsUp.

If you think about it the EPA does "black box testing". They are only measuring the energy that goes into the box vs the distance the box moved. They don't care about any of the values reported by the gimicky dash of any vehicle (EV, or ICE). This methodology ensures that the resulting numbers can be compared between different vehicles as the testing is all identical.

 

[WindPowered]

I agree with WattsUp.

If you think about it the EPA does "black box testing". They are only measuring the energy that goes into the box vs the distance the box moved. They don't care about any of the values reported by the gimicky dash of any vehicle (EV, or ICE). This methodology ensures that the resulting numbers can be compared between different vehicles as the testing is all identical.

I also concur that's WattsUp's description is correct per the EPA testing.

However, while the EPA testing is certainly the most fair (as it simply measures energy in vs. miles driven), it's not the most intuitive for the average consumer, thus the confusion we're seeing in the forums. i.e. It allows objective comparison of any two vehicles, but most people don't understand what they're comparing.

This methodology also leaves open the possibility that companies can improve their MPGe rating by simply* improving the efficiency of their internal charger. This would indeed reduce the amount of electricity consumed at the wall which is better for your wallet, but wouldn't yield any increase in range despite what some people might think at first glance when looking at the MPGe number.

I really hope that someday as EV's become more mainstream and there's no longer a need to compare them to ICE cars, that we can switch to Wh/m as the real measurement, but the EPA has set a precedent that I think we're just going to have to live with.


WP

* I say "simply", but this is in fact a hard problem and will only be improved slowly over many years as electronics and techniques continue to improve.

 

[DavidJohnson]

Hmmm, I can see I'm obviously wrong since everyone concurs. Thanks for the discussion/education and I'll calculate it your way from now on. :mrgreen:

 

[Arthur]

...actual charging efficiency isn't always 80%. It will be less efficient charging a very empty or an almost full battery, and will be less efficient if you are using plug energy to manage battery temp or precondition the car. So if you do those things, and want a more realistic idea of your MPGe, use a lower charging efficiency like 75% in your calculation.

I actually checked to see if there was a difference in charging efficiency between each of the 4 quarters of the charging cycle (as indicated by the blue display around the charging port). There wasn't. My FFE drew almost exactly the same amount of energy, in each quarter of the charging cycle, corresponding with an efficiency of between 72% and 73% (using the 120-volt charging cord that comes with the car).

As for where the lost energy is going, EV manufacturers seem to feel no need to share that information with their customers. The only thing I know for sure is that the energy lost inside the battery cells, as they are charged, would be less, for a slower charging rate. Since the opposite seems to be true of all modern EVs, the dominant source of inefficiency has nothing do with the battery cells themselves.

 

[Arthur]

...They (the EPA) don't care about any of the values reported by the gimicky dash of any vehicle (EV, or ICE). This methodology ensures that the resulting numbers can be compared between different vehicles as the testing is all identical.

It's true that the EPA doesn't care. They pretty much just "rubber-stamp" the data given to them by the auto manufacturers.

That said, the numbers on your dash are actually quite accurate. You can verify this, through several steps of observation and deductive reasoning, during the charging and discharging of your battery.

The numbers that are "gimmicky" (and misleading) are the ones presented on the window stickers of new electric vehicles. Even if we pretend that it is okay to give us MPGe numbers that are based upon a "wall-to-wheel" energy analysis (and that it is okay for them to NOT tell us that that is what they are doing), that doesn't change the fact that that energy analysis gives different results for 120-volt charging and 240-volt charging. When were they going to tell us which charging method the stated MPGe values (on the window sticker) correspond to?

...and then there is the "kWh/100 miles" number (32 kWh/100 mi, for the FFE). Is there some reason that the FFE displays Wh/mile, while it's window sticker displays the "kWh/100 miles" number? What are we to think, if we somehow, coincidentally, achieve 320 Wh/mile and see that number displayed on our dashboard? If we are capable of unit conversions, we might think that we have achieved the same fuel economy that is listed on the window sticker (and, as a result, we might expect to achieve a range of 76 miles, while achieving 320 Wh/mile). After all, that same number (only in slightly different units), is reported alongside the "76 miles of range" number (with no explanation that the 76 miles of range cannot be achieved, unless you simultaneously achieve a fuel economy of between 250 and 260 Wh/mile, as displayed on your dashboard).

Can anyone blame me for suspecting willful deception, especially when this inconsistency serves understate the MPGe (which is in Ford's best interest) and to OVERSTATE the range (which, of course, is ALSO in Ford's best interest.

 

[jmueller065]

Can anyone blame me for suspecting willful deception, especially when this inconsistency serves understate the MPGe (which is in Ford's best interest) and to OVERSTATE the range (which, of course, is ALSO in Ford's best interest.

Except that the calculation is done exactly the same for all the other manufacturers so the bias is present when comparing all window stickers against each other--therefore it cancels out and you CAN use those values when comparing between vehicles. Which is what those numbers are for: shopping; determining which vehicle you want to buy. I really don't think there is any willful deception: The EPA gives the manufacturers instructions on how to run the tests and calculate the numbers; it then verifies the data that it gets back from the manufacturers before its published...

I contend that you would be having this EXACT same argument on the Leaf forums had you purchased a Leaf instead of a Focus Electric...

 

[WattsUp]

That said, the numbers on your dash are actually quite accurate. You can verify this, through several steps of observation and deductive reasoning, during the charging and discharging of your battery.

Nobody ever said the displays (at least in the FFE) weren't accurate, just that what they show is of no consideration to the EPA.

Even if we pretend that it is okay to give us MPGe numbers that are based upon a "wall-to-wheel" energy analysis (and that it is okay for them to NOT tell us that that is what they are doing),

I don't understand why you think anyone is "pretending" here. The EPA is informing us how much energy per mile must actually be purchased to operate the EV. Would you prefer they ignore the total amount of energy required to charge the EV? That would make the operating cost appear to be much lower than it truly is. Would that not be considered misleading as well?

that doesn't change the fact that that energy analysis gives different results for 120-volt charging and 240-volt charging. When were they going to tell us which charging method the stated MPGe values (on the window sticker) correspond to?

The EPA's numbers seem to imply that they average charging efficiency they must have experienced was about 80%. This figure agrees with other available information about Lithium-ion charging efficiency, so I'm not terribly surprised by it. (In fact, I'm reassured.)

Also, the EPA is only attempting to provide an indication of average performance (and they even provide individual "city" and "highway" averages) to give potential buyers an idea of what to expect from any given car. I believe they even qualify their results with a statement to the effect of, "your mileage my vary". What would you prefer they do?

...and then there is the "kWh/100 miles" number (32 kWh/100 mi, for the FFE). Is there some reason that the FFE displays Wh/mile, while it's window sticker displays the "kWh/100 miles" number?

The kWh/100 miles rating is simply a normalized ratio the EPA happens to use. It appears on all EV stickers. Ford also happened to display energy usage as a multiple of watt-hours/mile in the FFE. They, or any other EV manufacturer, could just as easily chose to display energy usage in miles/watt-hours, or some other ratio.

What are we to think, if we somehow, coincidentally, achieve 320 Wh/mile and see that number displayed on our dashboard? If we are capable of unit conversions, we might think that we have achieved the same fuel economy that is listed on the window sticker

I agree, it can be confusing, but understanding EV energy usage is more complex than with a traditional ICE. With an EV, you simply need to be aware that some of the energy "used" by your vehicle occurs while charging, in addition to while driving. There are multiple aspects to energy usage with an EV, all of which impact the operating cost. The black box testing by the EPA takes them all into account and an educated EV consumer simply needs to understand this.

You are entitled to complain that you feel this is non-obvious, or difficult for the average person to understand right away, but it doesn't change the fact that the information reported by the EPA about the FFE, as far as any of us has been able to tell, is anything but absolutely correct. Indeed, most of us are able to achieve as good or better performance from our FFEs as the EPA's testing predicted we would. Although there's a learning curve, I think most of us are also able to drive in a manner that we expected (some city, some highway, at normal highway speeds).

At least that's what I expected. If you expected to be able drive 85 mph everywhere, with the AC or heat full on as you see fit, and still get 76 miles of range, then I can see why you might feel mislead. But, this wasn't because the information you were presented with was incorrect.

 

[Arthur]

Can anyone blame me for suspecting willful deception, especially when this inconsistency serves understate the MPGe (which is in Ford's best interest) and to OVERSTATE the range (which, of course, is ALSO in Ford's best interest.

Except that the calculation is done exactly the same for all the other manufacturers so the bias is present when comparing all window stickers against each other--therefore it cancels out and you CAN use those values when comparing between vehicles. Which is what those numbers are for: shopping; determining which vehicle you want to buy. I really don't think there is any willful deception: The EPA gives the manufacturers instructions on how to run the tests and calculate the numbers; it then verifies the data that it gets back from the manufacturers before its published...

I contend that you would be having this EXACT same argument on the Leaf forums had you purchased a Leaf instead of a Focus Electric...

You're right. The numbers are somewhat useful for comparing different cars, but not entirely. Unlike some claims made here, the reality is that NOT all cars have a charging efficiency of 80%. That's a good ballpark value, but, for some cars, the charging efficiency (on the same kind of charger) is a little higher or lower than 80%. Based on calculations alone, the upcoming Chevy Spark appears to have a charging efficiency of at least 86%. ...so, it still follows the pattern of having an artificially low MPGe and an artificially high range.

So, you're roughly correct, when you say that all of the manufacturers are playing this game. ...and that my criticism is independent of the brand I happened to choose to purchase.

 

[WattsUp]

The numbers are somewhat useful for comparing different cars, but not entirely. Unlike some claims made here, the reality is that NOT all cars have a charging efficiency of 80%. That's a good ballpark value, but, for some cars, the charging efficiency (on the same kind of charger) is a little higher or lower than 80%. Based on calculations alone, the upcoming Chevy Spark appears to have a charging efficiency of at least 86%. ...so, it still follows the pattern of having an artificially low MPGe and an artificially high range.

No, jmueller is exactly correct. The EPA's numbers are entirely useful for comparing vehicles. That is their sole purpose.

The ratings from the EPA are intended to enable shoppers to compare the economies of any two vehicles. For example, Car A with greater charging efficiency than Car B, assuming all else being equal, will receive a lower Wh/mile rating, and a higher MPGe rating, and will thus cost less per mile to operate. Or, Car A with greater motor efficiency than Car B, assuming all else being equal, could result in exactly the same Wh/mile difference as in the former scenario, with exactly the same economic disparity for the consumer.

Any specific difference in charging efficiency (or any other type; motor efficiency, wind resistance, etc.) between Car A and Car B is irrelevant. The EPA does not care about such specifics, they care only about the generalized result which, in the case of EVs, is simply "total kWh consumed versus miles traveled".

Think for the moment... The EPA has never cared about the specific efficiencies in comparing traditional ICE vehicles (engine efficiency, wind resistance, transmission losses, gasoline evaporation, etc.). Rather, they just measure "total gallons consumed versus miles traveled" and compute the MPG, which represents the total cost to operate the ICE (fuel-wise, anyway). They evaluate EVs using exactly the same principle.

It is the total operating cost, the overall economy, of any given vehicle that the EPA's numbers enable you to compare in a completely generic manner. That is the goal of their black box testing... for EV, ICE, or any other type of vehicle.