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Check my usage math

Discussion in 'North America' started by efusco, Jul 21, 2012.

  1. efusco

    efusco Moderator - Model S & X forums

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    Found a source that estimates the Model S will use 300Wh/mile (is that an accurate real world estimate?).

    I drive about 15,000 miles annually in my Prius. There will probably be fewer long trips in the Model S, but also more local trips, so I'm assuming similar annual mileage.

    If my math is accurate that's 300Wh/mile *15,000 miles = 4,500,000 Wh or 4,500kWh per annum and 375kWh/month.

    At $0.11/kW that's about $41.25/month in juice.

    Just trying to estimate what my total energy use needs will be for my home in anticipation of Solar City coming to give me an estimate.

    Any of my math or assumptions bad?


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  2. Citizen-T

    Citizen-T Active Member

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    For one thing you should see if your power company offers time of use pricing and think about at what time you will be charging. For me, and I figure that 95% of my fill up will happen at about 5¢/kWh.

    I didn't check the rest of your assumptions but your cost per month does seem to high to me.

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  3. Thumper

    Thumper Member

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    Just on the face of it, the 300wh/mile must be a little high for 55mph but is probably right for some higher speed. 300 miles (85kwh batt) times 300 wh/mile would require a 90kwh battery capacity which we know is too high. You are also unlikely to AVERAGE such a high watts/hr rate.
     
  4. richkae

    richkae VIN587

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    Charging is only 92% efficient, so 300 wh/mile is probably a good number. This is a calculation of kWh from the wall, not from the battery.
     
  5. brianman

    brianman Burrito Founder

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    There's a great opportunity here for Tesla and SolarCity to partner together to make "Model S powered by Solar" calculator web page. They could take as inputs your daily driving, sunshine stats for your geography (from your longitude / latitude), etc.
     
  6. Robert.Boston

    Robert.Boston Model S VIN P01536

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    I think that's a good working number, Evan. Of course, given how much more fun you'll have driving the Model S, you might need to tack on an extra 2,000 miles or so. :biggrin:
     
  7. Vger

    Vger Active Member

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    300 Wh/mi is a reasonable average. If you drive a lot in town and thus at 30-50 mph, you might often average only 250-280 Wh/mi. On the other hand, during my test drive, I noticed that the recent history on the dash indicated the non-perf car was averaging over 500 Wh/mi., with enthusiastic test driving. Caveat emptor.
     
  8. smorgasbord

    smorgasbord Active Member

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    The EPA says you'll get 265 miles on 85kWh. That means 321 kWh/mile, including charging losses. Maybe less since Tesla may not pump 85 kWh into an 85 kWh battery (they go to 95% full in Range mode). In that case you're looking at 305 watts/mile.

    So, you're close enough for this back of the envelope calculation.

    At 40MPG and $4/gallon, the Prius costs about $125/month. So, you're at about a 1/3 of the fuel costs.
     
  9. jerry33

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

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    40 mpg is very low for a Prius--unless you're always driving between 90 and 105 mph.

    2004 Prius MPG from the logbook. (Complete years only):

    2003-2004 -- 50.8 mpg 17,628 miles
    2005 -- 52.6 mpg 14,688 miles
    2006 -- 56.3 mpg 16,174 miles
    2007 -- 57.3 mpg 18,384 miles
    2008 -- 59.9 mpg 21,755 miles
    2009 -- 61.4 mpg 16,177 miles
    2010 -- 65.2 mpg 12,134 miles
    2011 -- 66.9 mpg 11,272 miles

    2012:
    DATE__________ODO____INC_____AVG
    --- Trip to NE starts here
    01/07/12____128603____481____56.6 (4.2)
    -- 13 F here
    01/12/12____129042____438____52.7 (4.5)
    01/15/12____129420____378____50.3 (4.7)
    01/20/12____129094____481____56.2 (4.2)
    --- Trip to NE ends here
    01/31/12____130503____600____69.8 (3.4)
    02/23/12____131050____546____69.4 (3.4)
    03/07/12____131679____629____72.2 (3.3)
    03/23/12____132319____638____71.3 (3.3)
    04/12/12____132987____668____74.0 (3.2)
    05/02/12____133647____659____74.3 (3.2)
    05/18/12____134272____624____73.4 (3.2)
    --- Flat tire here
    --- High temperatures start here
    06/06/12____134907____634____74.1 (3.2)
    06/22/12____135557____650____74.2 (3.2)
    07/11/12____136210____652____74.3 (3.2)
     
  10. brianman

    brianman Burrito Founder

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    @jerry33 - You've probably said this before, but I forget the answer.

    What accounts for your increasing efficiency? Improving driver efficiency skills? Some vehicle upgrades?
     
  11. roblab

    roblab Active Member

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    We have a Prius. Our average mpg is 40.

    We do not drive between 90 and 105, but at 35-55.

    We live at 1500 feet elevation, so our everyday trips require several trips downhill with very poor regen, and then a hard hauls uphill with very poor mileage.

    With very much care, the best we have done is 65 mpg, except in Britain we got 73 on vacation, but I wonder if it's calculated on the Imperial gallon there. But it doesn't happen driving around these mountains..
     
  12. jerry33

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

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    - The first 12 to 18 months I attribute mainly to new car break-in.

    - The 2012 increase is mostly because for the first time my commute changed since 2000. It's now 25 miles one way rather than 20 miles. That gives five more miles in the most efficient zone. Because the gas engine needs to warm up before you can get in the most efficient zone, it's pretty obvious that the longer any particular drive is, the less of a percentage the warm up time will be.

    - The rest is a combination of learning how to drive the Prius, practice, Scangauge (Oct 2007), and better tires.

    Each model Prius requires slightly different tactics to get the most out of it. It took the Prius group about three years to figure out everything and some things that were thought to be true in the early days have been dropped as being incorrect. The 2001 that my wife drives is just enough different that you can't use all the same methods in the same way. The 2010+ requires some different tactics as well but I don't have any experience with the latest model.

    Although much maligned by some in TMC, with the PiP 120 mpg shouldn't be hard to do. The thing about the PiP is that it appears to have been designed to overcome the shortcomings of the other Prius models. That is, if you drive it like a Volt--use the battery until it's gone and then switch to hybrid driving--you're missing probably 80% of the point. In my opinion (and I haven't driven one, so it's just my opinion) The extra battery is best used to overcome the weak points of the Prius:

    1. Driveway shuffle. Having the car start during driveway shuffling is a real gas waster (The gas engine starts seven seconds after power-on).

    2. Short trips to the store. The Prius will never warm up so mpg is poor.

    3. Really bad stop and go traffic. After about 10-15 minutes (maybe even less), the gas engine will run to keep the battery charged. This is really annoying when you're stopped or barely moving in traffic.

    4. The stop and go between your house and the main road. Although this varies by person, warmup is faster on main streets. During the first part of the warmup the Prius operates in EV mode (even though the display shows the engine supplying power to the wheels--the Prius display lies a lot and lags about three seconds). However, this behaviour is proven by the ignition timing of the gas engine because it's -5 during this time rather than a positive number.

    5. Other. There are a few other driving conditions where the engine can be inefficient for longer than the traction battery can handle.

    If you think about the PiP in this manner--it's a gas car with some electrics to reduce the gas engine inefficiency--it's not as bad as some folks pretend. It's not a fully electric car like the Model S but it isn't meant to be. And there are still plenty of people that have a distrust of batteries. For them, the Prius is a good start on the way to full EV acceptance. (Toyota really hates electric cars as far as I can tell, given the money they've spent to get rid of them by promoting anti-EV legislation and being against pro-EV legislation.)
     
  13. jerry33

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

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    #13 jerry33, Jul 21, 2012
    Last edited: Jul 21, 2012
    I'm in Texas, no imperial gallons here, so all my numbers (other than the l/100km numbers in parenthesis) are U.S. numbers.

    Certainly climate and topography have a direct bearing on the MPG of any car. In some cases there are ways to reduce the effect, in others there aren't.

    The U.K. uses imperial gallons.

    Some Prius group members find that elevation changes cancel themselves out. Of course, this depends, but on my trip to Arizona with about a 5,000 foot elevation change MPG was pretty good:

    DATE__________ODO____INC_____AVG
    --- Trip to AZ starts here
    12/04/08____85560____501____62.3 (3.8)
    -- 27 F here
    12/06/08____85929____368____53.4 (4.4)
    12/07/08____86322____392____56.7 (4.1)
    12/08/08____86800____478____60.8 (3.9)
    12/13/08____87232____431____57.4 (4.1)
    12/14/08____87673____440____63.3 (3.9)
    -- 25 F here and 50 mph winds
    12/17/08____88133____459____56.3 (4.2)
    --- Trip to AZ ends here
     
  14. efusco

    efusco Moderator - Model S & X forums

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    Thanks guys, glad to hear my numbers are realistic. Knowing that, we should be able to charge the S solely off the solar and still take some off of out regular electrical use and, thus, be able to afford the payments for the system on savings alone.

    My lifetime (9 years) average in my Prius hovers right around 60mpg, and gas is cheaper than most of the US in Missouri (currently $3.39), so my out of pocket savings that way won't be huge (that was never the point for me), but since the Prius will become my wife's daily driver and the Highlander hybrid relegated to special use (~27mpg average) combined with the Tesla savings, we should do very well.

    Our coop uses a $0.11/kWh regardless of time--Missouri is WAY behind on this stuff, no rebates for PV, nothing for EVs.


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  15. markb1

    markb1 Active Member

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    The 300 Wh/mile estimate comes directly from Tesla.

    Model S Facts | Tesla Motors
     
  16. Thumper

    Thumper Member

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    300 wh/mile then. I can learn!
     
  17. efusco

    efusco Moderator - Model S & X forums

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    Just for my edification, I calculated out a rough idea of what size PV array I'd need to offset my Model S.

    4500kW/yr divided by 365 days is 12.33kW/day
    Estimating 160W output/200W panel I'll need 12333/5 hrs/day (avg sunlight/day) = 2467W

    2467/160W (avg actual output from 200W panel) = 15.42 panels or just over 3kW solar system.

    Would probably aim for 4kW if my roof will accommodate it just for a buffer and to offset some of our home's normal usage.

    Math is fun!


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  18. AndyM

    AndyM Member

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    From earlier threads (too tired to search now, sorry) and posted research and graphs on this site, I had extrapolated these energy consumption rates for various speeds. I use these figures in a spreadsheet I set up a while ago before the "charging times" pages were added to the TeslaMotors web site (and displayed in the store).
    Due to wind resistance, mainly, energy use increased exponentially with speed.

    Speed - Wh/mile
    55 - 255
    60 - 275
    65 - 300
    70 - 325
    75 - 355
    80 - 380
     
  19. brianman

    brianman Burrito Founder

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    Andy - Please don't make the same mistake Tesla did. Please provide numbers up to 130 mph. :)
     
  20. AndyM

    AndyM Member

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    :cool: I'll be the last person to make up numbers, and don't want Tesla to do that either.
     

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