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Miles per kWh

Discussion in 'Model S: Battery & Charging' started by mknox, Jan 8, 2013.

  1. mknox

    mknox Well-Known Member

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    I just got my finalize button today, so I won't actually have my Model S for a couple of months. One of the things I'll have in place is a circuit meter on the NEMA 14-50 in my garage to measure the actual amount of energy the car is using. This would be power to charge the battery, power to replace "lost" vampire draw, power to heat/cool the pack... basically all the additional electricity I'll have to buy, for whatever reason, resulting from my ownership of a Model S. I'd then like to calculate my actual miles per kWh and electricity cost per mile. (My understanding is that the car's own systems only report on energy drawn from the pack, and doesn't take into account charging inefficiencies, pre heating/cooling and so forth).

    I'm wondering if anyone has done this and would share results. My guess is that the car actually is responsible for more electricity use than the Tesla advertised 300 Wh/mile (= 3.3 miles/kWh).
     
  2. Babylonfive

    Babylonfive Power12

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    I've been thinking the same thing, when I read about the 'pack heating' issues up in Canadia.
     
  3. Tesla 940

    Tesla 940 Member

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    My Roadster tells me the amount of electricity taken from the grid and the charging time (hours/minutes). By recording the beginning and ending Ideal Miles I can calculate the true cost. I have found that I lose about 25% - the majority of it from the conversion from AC to DC.

    I heard that the Model S is 93% efficient in charging but I don't know if that is for the SuperCharger / DC or regular charging / AC - or if there is a difference.

    I will find your information to be very interesting and look forward to seeing what others post. There are alot of smart people posting on this forum that seem "clueless" about the impact on charging inefficiencies and ghost loads that will probably have a huge impact on their cost to operate calculations. Knowing the true cost to drive per mile - in grid Wh - will be eye-opening.
     
  4. Zextraterrestrial

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    I should have some pretty solid data in a couple of months. Including temps and overnight losses (while unplugged) w/ hours of time and loss seen + all of my trip data(for almost every trip) Temps aren't freezing but I try to note If I have heat on / how much it uses. On my daily 4.1 mile trip to work my car is pretty warm by mile 1 and the heat pump drops off considerably.

    I won't be able to help with actual power pulled from the grid, just power 'used' for miles and 'vampire' chillin. (>90% of my charging has been on free 14-50 outlets)
     
  5. mknox

    mknox Well-Known Member

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    That's great to know. Perhaps Tesla can introduce something like this in Model S down the road. Based on your more temperate locale, 25% might be a good assumption to start with for Canadian spring/summer/fall conditions.
     
  6. Florent

    Florent Member

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    I have a "symhome" system installed at my house and I plan to monitor my HPWC with it... As soon as I get the car :p

    We've had the system since we got solar panels installed, it works fine (So far, the panels have been covering our power needs, I am hoping they will keep us at "net zero" even with what the Tesla is going to draw).

    That would have been nice to have some sort of cost monitoring with the HPWC or the car. Nothing too fancy, just an input for the local $/Wh and it would give out what you spent per month.

    I know it's a simple calculation and yes, I am that lazy.
     
  7. 100thMonkey

    100thMonkey Member

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    we have a blink charging station with a meter. I am tempted to leave it in place and use it for the S initially so that I can see real energy usage. it will be very interesting to see how much extra energy the battery warming uses. the best I've seen on the Leaf is 4.7 m/kW, but that was coming from the car dash.
     
  8. ElSupreme

    ElSupreme Model S 03182

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    I think this is a great exercise. I too am very interested in how much energy my car actually consumes. But I would stick to the energy per distance (Wh/mile or L/100km) units that Tesla is using (or km if you want full metric). Not the distance per energy units that we are used too in the USA (MPG). It is much easier to compare with distance in the denominator, and you get a better picture of what is actually being consumed.

    GPM - The MPG Illusion Website: Overview of GPM - Updated

    I think a huge advantage of moving to electric is to get away from 'bad' units like MPG.
     
  9. mknox

    mknox Well-Known Member

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    I tend to like MPG because I can relate to a gallon (or litre) of gas, how much it costs and then how far it will take me. Doing the conversion, we in Canada are paying about $4.75 / US Gallon. I know my current ICE gets about 22 MPG. Therefore I know that $4.75 will take me 22 miles. (I've lived in Canada for many years, and still don't like Metric). Distance in the denominator seems backwards to me. I never drive exactly 100 km. I take how far I drive, divide by MPG and know exactly how much fuel I'll use on that trip.

    But that's just me... whatever units people would like to use is fine; I can always do the math :smile: The point I'm trying to get at is what is the true, all in cost of energy to own, operate and drive a Model S.
     
  10. smorgasbord

    smorgasbord Active Member

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    EPA says Model S gets 89 MPGe, and that's measured from electricity coming out the wall to miles driven.

    I believe the conversion used is 36.6 kWh to a gallon of gasoline.
     
  11. mknox

    mknox Well-Known Member

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    That's what I'd like to verify. As they say "your mileage will vary" :wink:
     
  12. TXjak

    TXjak Owner/Investor/Advocate

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    I think a better comparison is cost/distance -- $/mile is pretty interesting.

    For example, a vehicle traveling 100 miles on 2.5 gallons of $3.20/g gas costs $0.08 per mile. A vehicle traveling 100 miles on 30 kWh of $0.10/kWh electricity costs $0.03 per mile.
     
  13. aronth5

    aronth5 Long Time Follower

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    I'm confused (again!)
    Are we sure MPGe includes the electricity coming from the wall plus miles driven? I thought mknox idea of metering what is coming from the wall would more accurately reflect the full cost of electricity (charging plus usage). What I see below relates to driving.

    http://en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent
    "The ratings are based on EPA's formula, in which 33.7 kilowatt hours of electricity is equivalent to one gallon of gasoline, and the energy consumption of each vehicle during EPA's five standard drive cycle tests simulating varying driving conditions"

    What am I missing?
     
  14. Doug_G

    Doug_G Lead Moderator

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    Pack heating will obviously increase the usage. That said, most people don't realize just how much extra energy their gas cars consume in extreme cold, so I wouldn't single out the Model S over that.
     
  15. gregincal

    gregincal Active Member

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    The question is what is meant by energy consumption. One imagines the EPA doesn't rely on the car's self reported energy consumption, but rather charges the battery fully, does the test and then measures how many kWhs it takes to charge the battery afterwards. However, as mknox said the point is to see if that's true.
     
  16. smorgasbord

    smorgasbord Active Member

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    The only way the EPA can measure is to fill up the battery, drive some miles, then fill up the battery again.
    The only way the EPA can measure how much electricity was used to drive those miles is at the wall. They are not going to trust any output that any manufacturer puts on its screens.
    When the 89 figure was announced, Musk mentioned that he wanted Tesla to redesign its HPC/UMC to yield better results.
     
  17. aronth5

    aronth5 Long Time Follower

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    Thanks. Will certainly be interested to see what the metering shows to reflect the true cost of electricity especially for those with cars in colder climates.
     
  18. mknox

    mknox Well-Known Member

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    Absolutely not! That's not my intent. My ICE cars obviously use more gas in the cold / winter but the difference is I know how much because I can see the litres and dollars right on the pump. I just want to be able to see the total amount of "fuel" that an EV uses in the same way (i.e. a meter on the NEMA 14-50 "pump").
     
  19. tezco

    tezco Sig P85

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    Funny you should ask. I was out in the garage today, working on that very idea. Unfortunately I got waylaid a bit when one of the torsion springs on my garage door decided to break last week, and had to spend a couple of hours today pulling out the old springs and tensioning up some new ones (don’t try this at home unless you know what you are doing….good way to break an arm, or your noggin…)

    Anyway, I'm in the process of assembling a small current and voltage measuring device to perform the task of recording my energy use. Nearly finished with the mods to the raintight box I’m using to house the equipment. I’m using a four-channel HOBO data logger which will record voltage, current and the air temperature at the floor of the garage (so I can get a temp reading at the level of the battery pack). Here is a photo of the hardware. (An easier way to do this might be to use TED devices; however, my garage is too far away from the house to allow for the in-wire transmission of data, and this will monitor voltage and temperature in addition to current...)

    HOBO.JPG
     
  20. brianman

    brianman Burrito Founder

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    I really wish Tesla would just do this in the car itself. I'm far more interested in the car telling me about its "energy profile and history" than about the ability to browse the web. I realize the web support is flashier, and thus why they were motivated to implement it first.
     

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