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Comparative efficiency with AC or with roof & windows open

Discussion in 'Model S: Driving Dynamics' started by Amaieseing, May 13, 2014.

  1. Amaieseing

    Amaieseing Member

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    Now that it's spring--finally--it would be great to drive with the roof and windows open, but I'm wondering if better range is achieved by closing up and using AC. &nbsp;Anyone know?<br><br><br>
     
  2. zwede

    zwede 2013 P85+

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    There's a Motor Trend video on youtube where they drove LA - Vegas before the superchargers opened and they mentioned that Tesla had told them that windows up, AC on, is more efficient than windows down, AC off. .
     
  3. dhrivnak

    dhrivnak Active Member

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    Not the case with the Roadster. We take nearly a 20% hit with the AC.
     
  4. Vger

    Vger Active Member

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    With the Model S, I think it depends on how fast you are going and what the temperature differential is. At highway speed the increased drag is significant. Below 50 mph, windows down might be fine. When I am trying to be very efficient, I leave the windows up and only air-condition to the minimum. But then again, I do not live nor have I travelled to the south!
     
  5. mnx

    mnx 2013 P85

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    Set the AC to off, drop the temperature as low as it will go and turn the fan up to 11. :) Works great for mild weather. No power draw from AC and no hit from decreased aero.
     
  6. ElSupreme

    ElSupreme Model S 03182

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    I regularly drive with all windows down, and roof open. At interstate speeds (I recommend using, and use myself, ear protection over 55mph with windows down) it uses much more power than the AC even on the hottest days. I think the breakpoint is ~45mph as to when the AC starts to be better (on very hot days). Stopping an going is better for windows down.

    Also the initial cool down of the car will draw a good amount of energy, but after that the AC really is only a small draw. And tinted windows help with AC draw a LOT!
     
  7. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    There is also a very big difference between starting after the car has been sitting in the sun (hot interior) and steady state. When you get into a hot car, opening the windows at the start can really help to get that heat out without pumping it out with the air conditioner.

    A sequence that I often use, that is comfortable and seems to be reasonably efficient for a green house heated hot car, is to drive for a few minutes with windows down to get rid of a lot of the excess heat, then turn on the AC and open the roof vent for many minutes more, letting the hot air in the cabin exhaust out the top while the AC fills the cabin with cool air, then closing up and going to AC only. My guess is that for medium outside temps, the break even point is more like 30-40 MPH in long term steady state conditions.

    The AC heat pump in the Tesla MS is very efficient compared to older ICE vehicles! It has its own variable speed compressor motor, etc, rather than a simple clutch drive off of the serpentine belt.
     
  8. scaesare

    scaesare Active Member

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    Windows, according to The Mythbusters

    That having been said, there are a number of forums where it's suggested that similar tests accounting for speed suggests that low speed = windows down wins, whereas high speed = A/C wins (for some value of "speed")
     
  9. mgboyes

    mgboyes Member

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    Your Questions Answered | Tesla Motors has an interactive gizmo where you can input your speed, the temperature outside, whether you have AC on, whether you have the windows open and whether it's day or night. I'm sure it's approximated but it should give you a good idea of the relative impacts.
     
  10. cinergi

    cinergi Active Member

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    Windows down isn't too bad. Sunroof, OTOH, puts a big dent in efficiency. A/C is definitely the most efficient for longer trips.
     
  11. jerry33

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

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    Opening the two windows opposite from you just a bit (20-30 mm) will bring the most breeze in with minimum wind resistance--speed doesn't seem to matter as you're not breaking the laminar flow. Next best is A/C.
     
  12. derekt75

    derekt75 Member

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    Interesting: even at 70 mph, windows down generally seems to be better than A/C. I wouldn't have guessed that.
    With A/C off, it's also interesting how much more range you get at higher temperatures (presumably due to decreased air density).
     
  13. ElSupreme

    ElSupreme Model S 03182

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    I can tell you any sort of prolonged travel at 70mph the AC is way better than windows + roof. Even at 95F outside.

    I regularly do both and I get 350+ Wh/mi doing windows down, and ~325Wh/mi AC blasting at 70mph.
     
  14. Amped-Up

    Amped-Up Member

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    Having the fan speed and the radio volume go to 11 was a stroke of genius. That's a 10% increase in power! Thanks Nigel.
     
  15. liuping

    liuping Active Member

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    I've heard a couple of times that AC is not counted in the Dash Wh/mi display. Is that possibly what happening?
     
  16. ElSupreme

    ElSupreme Model S 03182

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    The AC (and heat) both show up in your Wh/mi display, and on the instantaneous energy usage dial. I don't ever really use the energy graph so I don't know about that.

    What is happening is drag caused by the windows and sunroof is greater than the energy used to run the AC.

    Gas mileage hits - windows down vs sunroof vs A/C - Car Talk
    AC vs Windows : Discovery Channel
     

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