How do you calculate MPGE?

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How do you calculate MPGE?

Postby bowlofsalad » Wed Aug 07, 2013 11:01 am

Hello,

I have some guesses on calculating MPGE (mile per gallon of gasoline equivalent). According to various wikis, a gallon of gasoline might have 33.4 kwh (kilowatt-hours) or 33400 watt hours (lower end of figures I've found) of energy in it. That is a wild amount of energy, can you imagine having that much power in your battery pack?

Anyway, I simply take 33400(wh) and divide it by my watt hours per mile (wh/mi). So say you had a wh/mi of 20, 1670mpge is what I come up with when I divide 33400 by 20. Does this seem accurate?

Also, by this rationality, at around 15 cents per kwh comes out to about 5 dollars per 33.4kwh or per 'gallon' of electricity. We all talk about how cheap electricity is, perhaps were talking about how efficient our ebike setups are instead.

I would be pleased if someone corrected or enlightened me if I made any errors or totally misunderstand something.
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Re: How do you calculate MPGE?

Postby xenodius » Wed Aug 07, 2013 11:43 am

I calculate MPGe based on cost because that's what's useful to me. Car's don't get a lot of the total energy out of gas, so doing MPGe that way will always give you crazy high numbers.

So you get 250wh/mile, at 8 cents per kwh, that's 2 cents per mile. If you consider 20mpg in your alternate car, and $4/gallon, that's 20 cents per mile. So you get 200 MPGe. But that's just what I do to have a local cost-conversion so I can think in numbers I'm used to thinking with... mpg.
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Re: How do you calculate MPGE?

Postby major » Wed Aug 07, 2013 12:33 pm

bowlofsalad wrote:Also, by this rationality, at around 15 cents per kwh comes out to about 5 dollars per 33.4kwh or per 'gallon' of electricity. We all talk about how cheap electricity is, perhaps were talking about how efficient our ebike setups are instead.


Interesting viewpoint. So for simple comparison, say a gallon of gas costs $5. Which is the higher grade 'fuel' or energy storage medium? It depends on what your intended use is. For heat, both would yield about the same BTU. For transportation, the gallon of gas will convert to useful work of about 5 kWh whereas the electricity will get you about 25 kWh of work, or 5 times the distance. So on a value scale for your ebike, it is like paying $1/gal. I do a similar exercise with my EVcar and figure it is like paying $0.85/gal gasoline equivalent.
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Re: How do you calculate MPGE?

Postby xenodius » Wed Aug 07, 2013 4:24 pm

Keep in mind, however, that unlike fossil fuels you can harvest electricity in your backyard with solar panels. There's also hydro and nuclear sources... http://www.youtube.com/watch?v=uK367T7h6ZY

Electricity is also easier to transport.
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Re: How do you calculate MPGE?

Postby bowlofsalad » Wed Aug 07, 2013 7:06 pm

I think there may be a lot of confusion on this matter. In order to aid comprehension of these two ideas, try not to think about the size of your battery. Pretend your battery is the same size as a gallon of gasoline, and both hold 33400 watt hours. If a car goes 57 miles per gallon, you would divide 33400 by 57 to find it's watt hours per mile (wh/mi). In this case, it would come out to 585 wh/mi. If what I just said is true and accurate, then mathematically, would an ebike getting 20wh/mi be getting 1670mpge?

Perceptions and rationalizations aren't really relevant for this concept, math is. The question is, how can we calculate mpge?
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Re: How do you calculate MPGE?

Postby arkmundi » Wed Aug 07, 2013 9:59 pm

Its an interesting question. People use MPG to compare vehicles, which is more efficient. Then there's the http://www.epa.gov/fueleconomy/ standards to get progressive improvements. So when comparing an ebike running on electricity, its still about vehicle efficiency. I use the cost method to derive a number as its the most useful way of communicating some basic information. Burning a gallon of gas in a conventional ICE that gets, say 35 mpg, at $5.00/gallon ... what's the equivalent of that in ebike terms? So, how many miles will $5.00 of electricity get me? I've done some averaging and I'm using a rule-of-thumb of 1 amp-hour per mile, me on my bike on hilly terrain. Its currently taking me about 0.06 kwh per amp-hour to charge. I figure that by using a kill-a-watt meter during the charge cycle. I get the amp-hours from my watt's up meter at the end of a run. I'm paying about $0.19 per kwh for my electricity {because of my greenup option, 100% renewable}. Putting those numbers together, 1 mile costs me about $0.012. So $5.00 buys me 417 miles! :!:
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Re: How do you calculate MPGE?

Postby major » Wed Aug 07, 2013 11:09 pm

Where is the 20Wh/mi measured? At the output of the battery? At the wall socket for your charger? At the utility substation? At the power company generation station? At the fuel delivery port for the generation station? At the well or mine where the fuel is first obtained? It's a wide open question. And if you're charging your battery from a solar PV or wind turbine, what's a gallon got to do with it anyway?

I guess all that is why I relate back to the dollar. Compare the cost of gasoline per mile to the cost of electricity per mile.

And then: Are you using any human energy in that 20Wh/mi? How is that figured in? From the calories in your breakfast?
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Re: How do you calculate MPGE?

Postby xenodius » Thu Aug 08, 2013 12:06 am

I'm pretty sure most of us measure at the battery, but I think delivery is about 10 percent ( maybe less? ) and charging losses vary. It hardly makes sense to factor power plant and fuel product transportation costs into a measurement of your personal vehicles efficiency. If you want to track every electron, be my guest, but I want to know exactly how my vehicle compares to another vehicle. The efficiency of the grid is a whole separate issue that I don't care to combine with my vehicle efficiency specification. Either way though, the numbers are pretty beautiful. :-) 20wh/mile is thousands of mpg.
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Re: How do you calculate MPGE?

Postby dogman » Thu Aug 08, 2013 5:57 am

Pick a juicy round number like 100, then put it in your prius or leaf ad. Calculate? Nah, they just got somebody to drive slow enough to get that number.

If an actual calculation is made most I've seen use the gallon of gas costs x kwh of electricity costs y method.

Your calculation sounds more accurate to me. It shows the ebikes efficiency more accurately. Even my horribly inefficient longtail going 30 mph still gets 800mpg by that method. I get about 40wh/mi full boogie.

You are right, it's not that electricity is so cheap, it's that so much less of the wh is made into heat in electric motors. It's why you don't see big gas engines running stuff in factories. Big huge electric motors are much cheaper to run and maintain.
Last edited by dogman on Thu Aug 08, 2013 6:03 am, edited 1 time in total. View post history.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE

Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum

See battery technology section, FAQ thread at the top of the page for lipo noob info.

See the frankenbike longtail at the thread below.
viewtopic.php?f=6&t=28389&hilit=bouncing+betty
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Re: How do you calculate MPGE?

Postby arkmundi » Thu Aug 08, 2013 7:38 pm

dogman wrote:Your calculation sounds more accurate to me. It shows the ebikes efficiency more accurately. Even my horribly inefficient longtail going 30 mph still gets 800mpg by that method. I get about 40wh/mi full boogie.
You are right, it's not that electricity is so cheap, it's that so much less of the wh is made into heat in electric motors. It's why you don't see big gas engines running stuff in factories. Big huge electric motors are much cheaper to run and maintain.

Its also why ICE engines are so short-lived compared to electric motors, as well as have significant maintenance costs to keep them running. We have to advance to a TCO - total cost of ownership - model of figuring costs to get the real comparison.
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Re: How do you calculate MPGE?

Postby Dauntless » Tue Aug 13, 2013 2:35 am

100 miles driven x 33.7 kWh ÷ 34.0 kWh = 99 MPGe

http://www.epa.gov/otaq/carlabel/electr ... more.htm#2
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Re: How do you calculate MPGE?

Postby bowlofsalad » Tue Aug 13, 2013 8:46 am

Dauntless wrote:100 miles driven x 33.7 kWh ÷ 34.0 kWh = 99 MPGe

http://www.epa.gov/otaq/carlabel/electr ... more.htm#2


That math is weird. The watt hours per mile is right there, to find the watt hours per mile, I took 34 and divided it by 100, which gave me .34kwh or 340 watt hours (per mile). So in this case I would take 33700 (watt hours in a gallon of gasoline) and divide it by 340(wh/mi). Much more simple in my mind, and easier to reproduce or at least remember.
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Re: How do you calculate MPGE?

Postby Dauntless » Tue Aug 13, 2013 3:52 pm

Funny you bring that up, there are those who say that we should figure how much gas is used per mile. You're using all the same caculations, just in a different order. But that's an EPA style calculation I posted.

Oh, and about having that much power in your battery pack: That's the WHOLE REASON that electric cars are a struggle. You take out a 16 gallon fuel tank and put a 370 volt/1,500 amp battery in its' place, LiFePO4 that physical size couldn't cost all THAT much, I'd say you'd be able to build your LA to SF and back on a single charge car that some people could actually afford. In fact if the Tesla Model S really can go 300 miles with the 85kW battery, we'd be talking 2,000 miles, right?

How physically large would a 550kW battery have to be? What would it weigh? (Gulp)

http://www.teslamotors.com/forum/forums ... discussion
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Re: How do you calculate MPGE?

Postby bowlofsalad » Tue Aug 13, 2013 4:33 pm

Dauntless wrote:Funny you bring that up, there are those who say that we should figure how much gas is used per mile. You're using all the same caculations, just in a different order. But that's an EPA style calculation I posted.

Oh, and about having that much power in your battery pack: That's the WHOLE REASON that electric cars are a struggle. You take out a 16 gallon fuel tank and put a 370 volt/1,500 amp battery in its' place, LiFePO4 that physical size couldn't cost all THAT much, I'd say you'd be able to build your LA to SF and back on a single charge car that some people could actually afford. In fact if the Tesla Model S really can go 300 miles with the 85kW battery, we'd be talking 2,000 miles, right?

How physically large would a 550kW battery have to be? What would it weigh? (Gulp)

http://www.teslamotors.com/forum/forums ... discussion


We do a lot of conversions when we talk and think about things, personally I really dislike that idea. For example, some parts of the world use a measuring system other than metric, metric is so much better.

Perhaps we stick to watt hours because of how perfectly it defines our consumption per mile. Dividing into quantities of fuel make sense, but for most vehicles that use gasoline, the phrase miles per gallon fits them well. Speaking about traveling 100 miles and how much energy it takes though, it seems silly to me. .36kwh/mi seems like a strange thing to say. But, I suppose it might not really matter as those who understand probably won't be effected to much, the rest won't understand either way, hah.

Anyway, there is no doubt that battery energy density is a really, really big deal and likely the primary choke point that is preventing most people from considering using electric. I have spent some time marveling at how energy dense gasoline is and how pathetic the batteries used in EVs are today. What really blows my mind is the super tiny gain (comparatively) in energy density for lipo comes with a huge risk (compared to some other chemistry) but we go to that length for that little bump in energy density.

https://en.wikipedia.org/wiki/Energy_density This wiki does a good job at putting things into perspective. So many marvel at super capacitors, but look at their energy density, 2% of even our pathetic batteries energy density. Super capacitors aren't so super, in my mind.
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