TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

kW output vs Wh/mile vs speed?

Discussion in 'Model S: Battery & Charging' started by commasign, Jan 4, 2015.

  1. commasign

    commasign Active Member

    Joined:
    Aug 31, 2013
    Messages:
    1,058
    Location:
    Davis, CA
    #1 commasign, Jan 4, 2015
    Last edited: Jan 4, 2015
    Everyone talks about Wh/mile but that's something hard to adjust on a moment by moment basis (yes, I know you can look at energy usage screen but I like to keep that set to 30 mile average) . On the other hand, the kW output shown on the dash is instantaneous. Does anyone know how kW output maps to Wh/mile? Is there a simple formula or table to look at?

    I.e. ballpark guess, 30kW output, 75mph cruising speed at ideal conditions, 400Wh/mile in P85D (without torque sleep). Just a guess, haven't actually tested it because the heater is on and it's cold outside.

    On the power output meter, there is a big tick at 30, 60, 120, etc., but just below it is a small tick. I wonder if the small tick below 30 could be surrogate for rated range.

    Would also be helpful if we knew the translated range at each big and small kW tick mark.
     
  2. WarpedOne

    WarpedOne Supreme Premier

    Joined:
    Aug 17, 2006
    Messages:
    2,651
    Location:
    Slovenia, Europe
    Formula is simple if you know average speed or distance and time from which it can be directly calculated.
    Does model S display the average speed for some average Wh/mile?

    Say average speed was 40 mph and energy usage was 300 Wh/mile.
    Take an arbitrary length of time - say one hour.
    In one hour you would have driven 40 miles and used 300 Wh/mile * 40 miles = 12kWh of energy.
    12kWh used in one hour means 12kW average power.

    If your current speed was 50mph and current energy usage was 400Wh/mile, your current power output is 50 * 400 = 20000 W or 20kW.
     
  3. commasign

    commasign Active Member

    Joined:
    Aug 31, 2013
    Messages:
    1,058
    Location:
    Davis, CA
    I don't think the Tesla displays average speed for a trip (maybe I'm wrong, not at car right now). But the trip meter can be used to figure out how many kWh was used. So you could start a long trip, reset trip meter and log for exactly 1 hour to figure out your average kW output during that hour.
     
  4. WarpedOne

    WarpedOne Supreme Premier

    Joined:
    Aug 17, 2006
    Messages:
    2,651
    Location:
    Slovenia, Europe
    It is not necessary to log for exactly one hour. Any time duration will do as long as you have both - time and distance driven. You get average speed out of them and using "kWh used" counter you can calculate average power.
     
  5. Bighorn

    Bighorn Member

    Joined:
    Jun 19, 2013
    Messages:
    481
    Location:
    WY
    My gestalt is that 20kW on the highway approximates 300Wh/m. Terrain prevents any sort of scientific affirmation, but when I've done long distance trips and range was a concern, I'd aim for 20kW.
     
  6. smsprague

    smsprague Member

    Joined:
    Mar 3, 2014
    Messages:
    517
    Location:
    Tucson, AZ
    #6 smsprague, Jan 4, 2015
    Last edited: Jan 4, 2015
    Xxx
     
  7. Cottonwood

    Cottonwood Roadster#433, Model S#S37

    Joined:
    Feb 27, 2009
    Messages:
    5,062
    Location:
    Colorado
    The formula is very simple if you want to do a little mental math while driving.

    Energy per distance = power/speed​

    For example driving at 60 mi/hr and using 20 kW:
    20kW/60mph = 0.333 kW-hr or 333 W-hr​


    Mental math is good for the brain!
     
  8. evp

    evp Nerd

    Joined:
    Nov 28, 2014
    Messages:
    289
    Location:
    Arvada, CO
    Yeah, sometimes; not when you're overloaded and stressed. I found that I'm not nearly as good at mental math driving thru the middle of Wyoming at night in the rain, discovering that I can't trust the "rated range" indicator on my spanking new Tesla and needing to compute exactly how much it was lying to me. Especially since one of the key numbers (miles to destination) was too small to clearly read at night and at the bottom of the wrong screen.

    It reminds me of doing the same kind of worrying in a Cessna, then discovering that my new Cirrus has a remarkably accurate constantly updated "gallons at destination" display that cheerfully turns yellow or red if it feels that number is inadequate. The green arrow in the upper left of the picture points to a condensed analysis of the current flight plan. It says that we'll have 62 gallons remaining at our next waypoint (Goodland KS), but only 10 gallons left at our final destination (Duluth MN). Since we're burning 16.5GPH, that's underneath our recommended 45 minute reserve so the number is RED. Not the dim sullen magenta of the Tesla battery indicator; it's the bright "do something about this NOW" red that any number on the instrument panel is painted in if something bad is about to happen. We need this info box in the MS speedometer window whenever a destination is selected in Nav, or at least as an option in the left/right selectable areas.

    DSC_3591a.jpg
     
  9. commasign

    commasign Active Member

    Joined:
    Aug 31, 2013
    Messages:
    1,058
    Location:
    Davis, CA
    Lol, too much for my brain! I just wanted to know what kW output level to keep the throttle under such that the rated range on my dash is an accurate representation of actual range. Sounds like "under 20kW" is a good rule, though very difficult in the P85D right now at least until the torque sleep feature is added.
     
  10. jerry33

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

    Joined:
    Mar 8, 2012
    Messages:
    12,751
    Location:
    Texas
    Unless you are in the mountains, occasionally look at the energy graph projected range average 30 miles. If the projected range is greater than the rated range on the instrument panel, and there is more than enough rated range to arrive at your destination, you're fine. if there is less just slow down or find an alternative airport, err, charging location. Doesn't matter if it's slow as you'll only be picking up enough charge to make it to the next SC or HPWC.
     
  11. NOLA_Mike

    NOLA_Mike Active Member

    Joined:
    May 11, 2013
    Messages:
    1,556
    Location:
    Hammond, LA
    My experience is, yes, under 20 kW. This is made even harder on the P85D since the marked gradations changed and now it's 30 kW that is marked instead of 20 and I have a hard time determining where 20 kW is on the log scale...
     
  12. commasign

    commasign Active Member

    Joined:
    Aug 31, 2013
    Messages:
    1,058
    Location:
    Davis, CA
    Perhaps 20kW is the light gray smaller mark just below the 30kW mark.
     
  13. JohnQ

    JohnQ Active Member

    Joined:
    Jan 1, 2012
    Messages:
    1,585
    Location:
    Redding, CT
    Easy in an aircraft where you have calibrated fuel consumption at a given power configuration. Of course, the Tesla has that, too. As Cottonwood noted, travel at 60 mph at a constant 20 kw power drain and you're using 333 Wh/mi. Or, expressed differently, you have approximately 3.5 hours of fuel assuming a usable pack capacity of 70 kWh.

    The problem is that I can configure my aircraft for a constant power output but can't on the Tesla unless I want to have widely varying speeds traveling over lumpy terrain with varying wind speeds and surface friction. Not to mention barometric pressure variations that impact my wind resistance penalties. I appreciate what a good MFD can provide in an aircraft but it's serious overkill for the Tesla. I can modify my behavior quite well based on the rate of convergence of my rated miles on the speedo and the miles remaining to destination on my navigation. But I have to use common sense like recognizing that big-a** mountain at the end of my drive that requires something held in reserve.

    Would I like predictive range based on available statistics such as surface winds enroute and elevation profiles? Absolutely. Even better would be aggregated data from other vehicles that have traveled my same route in similar conditions and speeds. But that's future state. And the Tesla readouts don't lie. They simply need to be interpreted correctly.
     
  14. ken830

    ken830 Model S (Res#P12,447)

    Joined:
    Jun 19, 2012
    Messages:
    858
    Location:
    San Carlos, CA
    I think a little mental math would be fine. Current speed divided by power.

    A more practical approach I use is to calculate how much "reserve" I'm gaining or losing and adjust speed to compensate. Say, for example, I'm driving between the Tejon Ranch Supercharger and Harris Ranch Supercharger. Distance is 111 miles and I leave with 155 (44 miles of rated range "reserve"). Then after the first 10-miles, I do a mental subtraction (Rated Range minus Miles-to-Destination). If the rated range "reserve" is now 40-miles, I know I've eaten 4-miles of reserve. I know I'm just about okay because I can afford to do that 11 times (11 x 4 = 44) and still make it to my destination. If it's more than 4-miles lost, then I better slow down. If it's less, I can feel comfortable or even speed-up.

    Terrain and other road conditions need to be taken into account, of course.
     
  15. kennybobby

    kennybobby Member

    Joined:
    Sep 14, 2014
    Messages:
    479
    Location:
    Heart o' Dixie
    The large ticks double with each increment: 30, 60, 120, 240, 480.

    The small ticks just below also double with each increment: 25, 50, 100, 200, 400
     

Share This Page