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MPG or MPKW?

It struck me recently that when I transition to a Model S, it will not be rated by MPG (yea, I know, dah). Does anyone know how the mileage efficiency is measured on electric cars? Would appreciate your comments/thoughts on this.
Thanks

The EPA uses some formula, but the easiest thing to do is compare costs. It takes about 2¢ worth of electricity to drive one mile. At $2/gal, and getting 20 mpg, your present car would cost 10¢/mi. So that's 5X your present mileage.

But the quoted figure is around 240mpg equivalent. That must be using other factors, too.

The EPA sticker that came with mine gave the number in kWh per 100 miles. There's a couple of things that I like about that.

First, kWh is a common unit to measure energy, and it's easy to check the rate on your electric bill to figure out what it will cost you to drive the car.

Second, It's also energy/distance, rather than distance/energy as in mpg. Since most people travel a given distance a year and wonder how much energy they will use, it makes more sense to have distance as the denominator--then a varying numerator lets you easily compare cars with varying efficiencies in a linear fashion. Having energy as the denominator would make sense if you had a fixed amount of energy and wondered how far you could get, but I don't think many people find themselves in that situation.

(As an example, mpg numbers are why most people are more interested in bumping mpg in their commuter from 50 to 60, even though bumping mpg in their minivan from 25 to 30 would save twice as much gas. This would be easier to see if we used gallons per 100 miles, which would show the commuter rating going from 2 to 1.667, and the minivan rating going from 4 to 3.333.)

However, not everybody likes kWh hours per 100 miles. The biggest objection is that people used to driving gas cars have no idea what they're looking at. Even if they run the numbers and figure out what they are looking at, it is still hard to compare efficiency between a gas car and an electric car. Some organizations, like X Prize and Consumer Reports, recommend using mpge, where "e" means equivalent. It is exactly mpg for a gas car, but in an electric they figure out how many kWh are in a gallon of gas, and use that number instead. I think the Tesla gets something like 120mpge.

I see advantages to both. Mpge has good short-term advantages that might help get people in to electric cars. But long-term, I think kWh/100 miles is going to be easier to deal with and more useful in getting more efficient electric vehicles.

I can't speak to technical equivalences, at least not without some heavy duty googling, but when I was figuring out cost to drive for the S I compared it to the range on my current car.

I took the range of my car on a full tank of gas (noting the cost of a full tank) and then equated that to "charges" for the S. At the time I was going off of a 230mi battery pack. My current car gets approximately 420 miles to a tank of gas. Given that I'll be too excited to drive in a responsible manner with the S, I guestimated 2 charges for equivalence (or you could say I put in padding for less than optimal battery range).

From here, I was able to look up the time to fully charge from empty based on the different charging solutions and the cost of kWh in my area. This gave me the cost per charge with a simple set of calculations (if I recall correctly, the most expensive equivalence was like $12).

With these numbers, it would be easy to take it a step farther and assign miles to kWh in the same way one finds hours of laptop battery life based on their mAh ratings.

The caveat is that measuring efficiency of these cars in miles per kilowatt would have the same misleading representation as does measuring gasoline based vehicles in miles per gallon. Somewhere out there, an article exists explaining the split between mpg and fuel consumption.

I hope somewhere in my rambling there was a nugget of helpful information.

Cheers

The reason the EPA even provides MPG is mostly because they're dumb. There's no rational method to equate electricity and gasoline
Trying to equate in terms of energy (joules per gallon, etc.) is ridiculous, since 1) nobody gives a damn, and 2) it provides no meaningful information, as the two types of vehicles utilize
energy in very different ways. Using costs makes even less sense, since, as anyone can plainly see, there is no fixed relationship between the price of a kilowatthour and the price of a gallon of gasoline. There's not even any relationship between the price of a kilowatthour in one state versus another. In their attempt to both
make the consumer aware of the energy consumption of a vehicle and
use terms the consumer is familiar with (MPG) the EPA has managed to thoroughly obfuscate the entire matter. Any semi-intelligent elementary school student knows that miles per kilowatthour is how the energy consumption of an electric car should be expressed.
What, you expected intelligence from Washington?

I don't think it is EPA that is dumb, it is the people they are trying to serve that are dumb. People are used to see mpg-figures so EPA is trying to make meaningful efficiency calculation from kW to mpg-conversion.

That is not necessary very meaningful no matter how you put it, but it can be used to give some rough estimate on pollution figures in electric vehicles....that is until source of electricity is clean. Then all those mpge-figures can be thrown to the garbage can.

I sometimes need to remind myself that average IQ of people is only 100. (As it is by definition. Just don't expect too much from Joe Average).

Let's hope that with the Japanese nuclear accidents it doesn't mean my treehugger self is going to have to make do with more coal generated electric. Damn if I only bought solar panels instead of stock in evergreen solar...

That earthquake was a bad one, it looks like that the entire fault line is shifting there. Bad place to put nuclear plant. There is another very big earthquake coming in Tokyo bay that is already late (category 8-9 Richter, probably also causes massive tsunami just like this one, except epicenter will probably be even closer to the coast line).

Does anybody know where Panasonic is manufacturing those advanced batteries? Laboratories in line of fire? That would be bad for Tesla. Really bad.

I hope LPP experiment gets to the actual results soon. We need clean energy in form that can't cause accidents like this one, can be used everywhere all of time and is mobile.

Then we really could throw the MPGe figures out of the picture and start using MPkWh.

@Peak - you still can buy a PV array. I installed one four years ago (42 panels / 9.5k kWh) and can't tell you how happy I am with them. The ROI is probably 8 years with the CT incentives at the time and the energy costs. I plan on being in my home far longer than that, so it works for me. The economic value to living up to my own tree-hugger standards is priceless! Especially for those 9 months of the year I don't pay for any electricity at all!

According to http://www.teslamotors.com/about/press/releases/panasonic-presents-first... the Model S batteries will be built in a new facility in Suminoe, Japan. That's far to the west of Tokyo.

Milage should be in both miles per kWh and an explicit energy conversion for gas "equivalent."

Seems to be in well-protected place against tsunamis, earthquakes can obviously happen there too, but not that bad.

Unfortunately if I have understood the place of that predicted earthquake correctly it will be between Philippine and Eurasian plates which puts it very close to that place. What happened now was between North American and Pacific plates quite a bit north from Tokyo.

If Google Earth is to be believing one of the major ones happened practically inside Fuji (mountain, not the city). If it did something to the lava in Fuji that can be extra bad. If that does same as St Helens did that's major disaster.

I hope this doesn't cascade to something really bad. It is already bad, but it could have been much much worse.

MPG is a relic from the past, or soon will be. Our brainless EPA or DOT insists that Americans only undersatnd MPG and so they make these absurdly invalid "equivalence measures" to enable the feeble minded American consumer to figure out how efficient or costly this electric is to operate. Miles/kilometers per kilowatthour is the proper specification for mileage for an electric car. Why the Feds find this inappropriate is a mystery yet to be solved, except that they also think we can't stop piling up debt, else the economy might suffer.

Who cares what your mpge or mpkwhr or whatever is? So it costs you 2 cents per mile instead of 2.5? Big whoop. Range is all that really maters for a pure electric car. This is just a relic of ICE mentality.

In my current vehicle when I start it up it says MTE (Miles to Empty) which is usually pretty close to reality with all things considered like outside temp and how heavy my foot is.

For the Model S I think it's reasonable to know the following:

1. From my phone it should let me know max miles to empty if I drove at max efficiency taking into account current and future outside temp. And expected miles to empty taking into account normal driving habits.
2. From my phone it should let me know if I can make a destination and get back based on MMTE and EMTE from #1. And if not, how many more minutes of charging will it take.

I'd list all the others but I believe they are all being implemented by Tesla.

@kevinf311:
"Given that I'll be too excited to drive in a responsible manner with the S"

That cracked me up. Thanks for the laugh :-)

I figure I'll drive in a responsible manner (I always do, I'm the most cautious driver I know).... but for the first several months at least, I'll probably start taking joyrides through the countryside instead of just driving when I have somewhere to go. :-) So I know what you mean.

If you think you are "cautious driver" get a Yello Flag -CD and play "the Race" while driving. You soon realize that "cautious" is relative term and speed limits are more like guidelines than rules.

;-)

Seriously though, I never speed, though I do have real life "heavy foot" so that if I don't pay attention to speedometer I easily drive too fast, especially in a car where speed doesn't feel. One of my friend had a bit sporty car with speedometer light broken in 80s. I drove it (designated driver for that night). When he asked me to slow down a bit I just wondered "why" and he then used a lighter to give light to speedometer. I was doing 120 in 80 speed limit. Without realizing. I wonder how fast I would drive in a car where I can't even hear the engine....

Unless the sound insulation is ridiculously good, you'll probably still be able to hear some gearbox sound.

I find that in the Roadster I can tell my speed better than in an ICE car, because I can hear a slight whine from the gearbox.

I find that in the Roadster I can tell my speed better than in an ICE car, because I can hear a slight whine from the gearbox. (Douglas3)

I had the same experience when I recently test drove the Roadster. At least during the test drive, I liked the sound, which becomes especially audible when accelerating. I hope that they do not entirely insulate it away. But then again, it may become annoying when it is always present. I don't know...

Back on subject, I believe the reason we currently use MPG is because people want to equate what they buy to distance traveled. eg. How many miles can I drive on the 10 gallons of gas I purchased? Makes sense.

People need to know that their cost to drive an EV is dramatically lower than their cost to drive an ICE vehicle. How will they know that? How can they compare? If you start talking about miles per kWh, you might as well talk to the wall. It will do nothing towards getting the regular guy in an EV. But if you tell them "You were getting 25 MPG, but now you will get 120 MPGe", you will get their attention. Then tell them they can drive about 300 miles on about $10 of electricity, and they will be sold. If they are still not sold, tell them they can drive all month for what they are currently paying for gas every week. They'll understand that.

It will be quite a while before anybody (in the USA) who is not a scientist, electrician, or some sort of engineer really understands miles per kWh.

I agree with David M.

I understand kWh but not everyone is a 'nerd' like me.

Some of the average Joe's I wirk with have a hard time with simpler things.

When I look at the costs I use a similar method to kevinf311.

Compare how many miles your current car gets per gas tank, note how much a gas tank costs you.

Take your favorite battery pack option, look up how many kwh it uses, multiply that by the cost of a kwh on your latest utility bill.

Compare.

I came up with around 2 cents a mile for an electic car and 8 cents a mile for gas.

Obviously this would change if I drove a more efficent ICE car, the price of gas fell, or my electricity went up. However, those are real costs for me, today. If the price of one of those variables changes I can always look at it again.

My only problem is I cant afford the Model S. I am of the crowd that must wait until things come down a bit.

viao,

thanks for your contribution. Sorry you cannot afford a Model S right now. It is the Model S customers that will help bring affordable EVs into the market. In the time between, help spread the word to every Joe Sixpack. You found great words to spell the EV magic!

Forget the EPA - they are pushing this MPGe number, of their own making, which, for some bizarre reason, they think will make it easier for consumers to compare mileages. MPKwHR is , of course, the
figure we want - miles per kilowatthour. Everyone knows what they pay for electricity, and therefore can easily figure out the cost per mile. For extended range vehicles like the Volt , we need MPkWhr and MPG, for mileages when running off the battery or range extender
engine.

Actually, I was convinced that we've got the numerator and denominator mixed up by this great article: http://physics.ucsd.edu/do-the-math/2011/08/mpg-for-electric-cars/

In relevant part:
"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)."

The author proposes that the metric should be kW/100-miles. "A gallon of gasoline contains 36.6 kWh of heat energy when combusted," so we can compute the kW/100-miles for an ICE or hybrid as 3660/MPG.

Seems more relevant to use MPD (Miles per Dollar) as all my calculations all come back to how far I can go for a single greenback.

Again, I think you've got that flipped around -- I care more about how much it's going to cost to go to a particular place? E.g., if I want to go skiing on the slopes 170 miles away, how much will it cost in my Audi vs. Model S?

I don't know that most of us care too much about what the cost to go 50 miles will be. In the end, the cocktail party conversation will be about how much it costs in comparison to the listeners' ICE car.

My comparison will be the equivalent miles per gallon as computed using kWh per mile, $/kWh, and $/gallon. The computed equivalent will make it evident to ICE drivers that I'll be using much less than that Prius when I take to the HOV lane. And the "green" vogue will make the S more desirable.

Really: how many miles would I have to put on my S to recoup the extra cost over buying the ICE car I would otherwise buy? I'm fooling myself if I think it's all about saving money. It's about a social change toward efficiency and cleanliness and leaving the smoggy past behind, without giving up the fun (this last is to be the legacy of Tesla).

Robert.Boston, the mix-up may be rooted in the fact that Americans highlight how far they can go on a fixed amount of fuel (1 gallon), while in Europe people usually ask how much fuel they need to go a fixed distance (100 km). Google Guru does a great job converting the two units, e.g.,

"20 mpg to l/100km"
-> "20 miles per gallon = 11.7607292 l/100km"

On top, the European test cycle is different from the EPA cycle. For instance, the 528i is EPA-rated 22 mpg(City)/32 mpg(Highway), while the European rating is 6.8 l/100km. The plain mathematical conversion yields:
22 mpg = 10.7 l/100km
32 mpg = 7.4 l/100km

Note I'm not saying anything why or how the article is right or wrong. Just stating that fuel consumption is expressed with nominator and denominator swapped in Europe vs. USA.

One thing every one has to remember:

1 US Gallon = 3.785 liters

1 UK gallon = 4.55 liters

As of today:

1 gallon of US gasoline = $3.33 (as of today, 11,16/2011, MD, USA)

1 Kilowatthour = $0.136040146 (Inclusive of all charges, taxes, etc in Maryland)

Therefore:

($3.33 / $0.136040146) = 24.781 Kwhrs

24.781 Kwhrs * 1000 = 24781 watthrs

(24781 watthrs) / (300 watthrs/mile) = 82.6 miles

As in a ICE vs an EV engine, the bottom line will be dependent on tire pressure, road conditions, driving habits, and other factors.

One thing. Got it. Thanks.

Tesla sales? You might want to use this calculation in each of your US stores (change appropriately for other areas), just to let customers get some idea of where they stand when they want to know how much it costs to drive an EV.

[ Constants (like 300 watt-hours/mile) used here should be changed as per Tesla numbers and local gasoline units; those used here are near forum USA best guess only ]

"Using local electrical Rates and prices for gasoline (Dollars per gallon)"
Re (Rate for electricity) is in dollars per kWh
Dg (Dollars for a gallon)

(1000/300) * Dg / Re = Equivalent MPG

Great suggestion EdG!

Hopefully you suggestion will fly. It will help sales staff get more people to consider the benefits and not just look at the sticker price.


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