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How to calculate the cost to charge?

Discussion in 'Model S: Battery & Charging' started by Tessier, May 22, 2012.

  1. Tessier

    Tessier Member

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    I am trying to figure out how calculate what it would cost to charge an 85 kWh battery from empty to full. The general idea is what would it cost to run a model S for 300 miles. View attachment Summary_of_Rates.pdf I have attached my local utilities rates but every equation I use seams to come out with outrageous numbers. I was hoping someone to in tell me how you calculate and compare model S to a ICE.:cursing:
     
  2. bonnie

    bonnie Oil is for sissies.

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    What are you assuming is your cost per kWh?
     
  3. ChadS

    ChadS Petroleum is for sissies

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    #3 ChadS, May 22, 2012
    Last edited: May 22, 2012
    Just multiply 85kWh * your electric rate. If you get the R-OTOD plan, it looks like your off-peak rate would be about 11.3 cents/kWh, so you'd be about $9.60. Plus maybe 10% as Todd points out below for charging losses.

    I think a more useful comparison is to calculate monthly fuel savings based on how far you drive. Here's an example from Plug In America's "Electric Driveway" party kit:

    Tips on Calculating Fuel Savings

    Most electric passenger vehicles will travel 3-4 miles per kWh, depending on efficiency and driving style. The US average kWh costs 11 cents. So a plug-in vehicle can travel a mile on 2.75 to 3.7 cents. The average driver drives 1,000 miles per month, and so will spend between $27.50 and $37 on electricity each month.

    The average gasoline passenger vehicle gets 22mpg. The US average cost of fuel at the moment is $3.50. So gas costs 15.9 cents per mile, which would be $159/month.

    The average driver would save over $140/month in fuel costs if they switched to electric drive. That’s in addition to savings on maintenance, oil changes and emissions inspections. And based on past pricing trends, gas prices are likely to rise significantly more than electricity prices, so the savings are likely to grow.

    To calculate this for a specific instance, you need to know
    • The price of a kWh of electricity from your utility
    • The price of a gallon of gas
    • The MPG of the old car (see fueleconomy.gov)
    • The kWh/100 mile rating of the plug-in vehicle (fueleconomy.gov just started listing these too)
    • How far you typically drive in a month

    Plug these numbers in to the following equations (with average samples given):

    Gas price per gallon / Miles per gallon = Gas price per mile
    $4 / 22mpg = $0.182

    Electricity price per kWh * kWh per 100 miles / 100 = Electricity price per mile
    $0.11 * 34 / 100 = $0.0374

    Gas price per mile – Electricity price per mile = Savings per mile
    $0.182 - $0.0374 = $0.1444

    Savings per mile * Miles driven per month = Dollars saved per month
    $0.1444 * 1,000 = $144.40
     
  4. Todd Burch

    Todd Burch Electron Pilot

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    It's complicated if you want really accurate numbers...there are auxiliary losses, you're not filling up the entire 85kWh of the battery, etc...but for a ballpark figure, use:

    Cost per kWh for electricity * (85 kWh / charging efficiency)

    Example:
    10 cents per kWh
    90% charging efficiency

    0.1*85/.9=$9.45

    I don't recall the actual charger efficiency but it's somewhere in this range. This equation should get you within a dollar or two of the cost.
     
  5. smorgasbord

    smorgasbord Active Member

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    EPA 5-cycle ratings indicate 3.1 miles/kWh. Assume a 90% charging efficiency and you're at 2.8 mile/kWh.

    So, perform the following;
    1) For Model S, divide your electricity cost/kWh by 2.8 and that's what Model S costs per mile.
    2) For ICE, divide your cost/gallon by your MPG to find that it costs per mile.
    3) Compare and smile.
     
  6. Lyon

    Lyon 2016 S P100DL, 2016 X P90D

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    That sheet threw me off for a moment until I read the units on the charges. It looks like it could be as much as $.15 per kWh so charging the model S 85 kWh battery from empty is around $13.

    Even if that only gets you 265 miles because you're flying down the freeway, that's still pretty cheap.
     
  7. Iz

    Iz EVs are here to stay

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    This is a very good thread. ChadS thanks for posting the Plug In America information.
     
  8. hj-45

    hj-45 Member

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    I spoke with a PSNH representative and grilled him pretty good. Bottom line is they do not have a rate structure favorable for EVs. They are developing something specifically for EV owners, but would still have to have it approved by the NH Public Utilities Commission.
    The Rate R is what mast people have, which does not differentiate between on or off peak. The Rate-OTOD does recognize time of day service; however, the meter is installed in ADDITION to your normal meter. So now you pay $11.54 for the normal meter and $26.80 for the TOD meter just to have them on your house. I calculated using this setup charging 250 miles/week and the cost was actually $4 more/month than just charging on my normal meter. It seems the Rate-OTOD is for apartments, multi-units etc. which utilize a large storage tank for the building hot water. They only heat the water at night and use enough juice to benefit. Maybe with the 85KWbattery, and more mileage, you would also benefit.
     
  9. Zextraterrestrial

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    #9 Zextraterrestrial, May 23, 2012
    Last edited: May 23, 2012
    $26.8 per mo extra just 4 that meter too + $11.54?? ouch
    Sounds like PG&E rates in Nor Cal

    I switched to TOU E-6 in March. Off peak is still $0.31/kWh >tier3 (peak $0.49) but we have the 'smart' meters installed and it cost money not to have them

    I almost always hit the 4th tier at least a little bit so I figure 0.31*85 for a full charge or ~ $30 for 260-300 miles

    compared to my Rav4 below(last bit is mostly city driving so mpg has been lower)

    rav4.JPG
     
  10. richkae

    richkae VIN587

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    It is not worth going to crazy effort to install additional meters unless you drive a lot - like more than 20,000 miles per year.

    The average house uses about 30-40 kWh per day.
    I use about 8 kWh per day to drive to and from work.
    But I managed to save more than 8 kWh per day by installing LED bulbs, fixing my HVAC ducts ( they were leaking and heating/cooling the crawlspace ) and aggressively turning stuff off ( children leave TVs they are not watching on for hours ).
    My household now consumes about the same electricity as it did before the EV because of this conservation.
    ( Or another way to put it, my conservation efforts made driving my car free )

    Leaving a plasma TV ( or a desktop PC and monitor ) on all day uses almost the same energy as it takes for me to do my daily commute.
     
  11. dsm363

    dsm363 Roadster + Sig Model S

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    Even when computer goes into sleep mode? I normally just leave it on but probably not best. Good to know. I'm leasing my house otherwise I'd go to all LED lights.
     
  12. Lyon

    Lyon 2016 S P100DL, 2016 X P90D

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    Not to get too far off-topic here, but I just replaced the 17 (yes, 17) 50watt bulbs in my kitchen ceiling with 7watt LEDs. They also produce less than half the heat (so less AC needed) than the regular bulbs. As bulbs burn out in my house I'm replacing them with LEDs or CFs.

    The point is, that with a little conservation, one can pretty easily offset the use of the Model S if one doesn't have a long commute.
     
  13. gg_got_a_tesla

    gg_got_a_tesla Model S: VIN P65513, Model 3 Res Holder

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  14. markb1

    markb1 Active Member

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    From Model S Facts | Tesla Motors

     
  15. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Never happens :smile:
     
  16. stopcrazypp

    stopcrazypp Well-Known Member

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    Small correction. All EPA numbers include charging losses already (because the kWh being measured is at the AC socket, not at the car battery).
     
  17. smorgasbord

    smorgasbord Active Member

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    Not in terms of range, which is just how many miles.

    I simply took 265 miles and divided by 85kWh to get 3.1 miles/kWh. Since I'm using the 85 kWh of the battery, I still need to account for the additional juice needed to get a fully charged battery, no? That's where the 0.9 factor comes in, right?

    Another way to look at it is 265 miles on 94.4kWh of juice is 2.8 miles/kWh. The 94.4 kWh is what you need to end up with 85 kWh in the battery.
     
  18. stopcrazypp

    stopcrazypp Well-Known Member

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    Sorry, you are correct if you are using 85kWh (the battery capacity) as the base number. It doesn't appear Tesla has released official sticker numbers, so I guess that is the best guess until then.
     
  19. smorgasbord

    smorgasbord Active Member

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    Elon said it would be 89 MPGe in the conference call, but then said they were going to tweak the charger (I assume the HPC, not the UMC, but could be wrong) to be more efficient and get that number up.
     
  20. VolkerP

    VolkerP EU Model S P-37

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    I hope they tweak the on board 10kW charging units. All AC charging would benefit from that, be it 110V, 240V, J1772, Mennekes, single or three phase AC.
    Right now the on board chargers are reported to have a 92% efficiency. I think that's rather poor, given that solar inverters approach 99%.
     

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