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Primer/Tutorial on kW, kWh, electricity/energy and ready those energy graphs?

Discussion in 'Model S: Battery & Charging' started by Soundchasr, Jun 21, 2016.

  1. Soundchasr

    Soundchasr Member

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    I don't really have a good understanding of a lot of the terms used when people talk about the batteries and charging like kW, kWh, etc. And I have no idea what those energy graphs in the car mean. Is there a good tutorial source to learn more? The manual isn't very helpfu. Thanks!


    Edit: Title "Ready" should be "Reading"
     
  2. Boourns

    Boourns Member

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    #2 Boourns, Jun 21, 2016
    Last edited: Jun 21, 2016
    There are others who can explain it much better than I can but the short answer is:
    • kW (kilowatts) is a measurement of how much energy the car is using at any given moment.
    • kWh (kilowatt-hours) is a measurement of how much energy is used over time.
    Your car's battery is either 60, 70, 75, 85, or 90 (this used to be so much easier!) kWh. Your driving habits dictate how long (or far) that fixed number of kWh will last. Accelerating hard and driving fast uses more energy at any given moment, which gives you less energy to use over time.

    The graphs show you the Wh you use per mile. So it's your consumption of energy per mile driven. Each data point on the orange line is the energy you used and the average of those points over 5, 15, or 30 miles is the Wh/mile over that distance. A Wh/mile of around 300 is what's used for the rated range that shows on the driver display.
     
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  3. jkk_

    jkk_ Member

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    Model 3 reserver pleb here - been also thinking about different units etc x years in advance (x depends on how optimistic one is).

    One tip regarding Wh/mile (or km) that feels natural for me (since consumption of ICE here is measured as x liters per 100km) is to transfer Wh/mile to kWh/100 miles.

    So:
    Feel free to educate and correct me if my thinking doesn't make sense. Like I said, just a pleb here.
     
  4. jerry33

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

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    This kind of depends on what your used to. For me X per 100 km/mi is just unnatural because you never travel exactly 100 km/mi, whereas if I divide how far I'm going by the mi/kw per gallon/litre or kW per km/mile, I'll know how much energy I'll use.
     
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  5. wdolson

    wdolson Active Member

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    It's kind of like the difference between miles and miles per hour. The former is distance and the latter is speed. In the case of energy and power, a Watt-hour is a measurement of the energy used. If you turn on a 100W light bulb and leave it on for 1 hour, you have used 100 Watt-hours of energy, but your instantaneous power usage at any point in that hour would have been 100 Watts. If you have only left the light bulb on for 1/2 hour, you would have used 50 Watt-Hours of electricity, but your power at any point would still have been 100 W.
     
  6. SageBrush

    SageBrush Active Member

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    Consider a stoppered sink of running water filling up:

    You have a flow of water
    You also have an amount of water that has accumulated

    The two are related through time: flow * time = amount

    Watt is a unit of flow. It is called power
    power * time = amount, called energy

    If the time is one hour, then the amount of energy is watt * 1 hour, abbreviated 1 Wh
    If the flow is 1000 watts for one hour then the amount of energy is 1000 Wh, also written as 1 kWh

    ---
    After you fill up the sink, you can turn off the in-flow and let it drain. That would be out-flow.

    See how batteries are the same ?
     
  7. Boourns

    Boourns Member

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    That is another way to do it. The Nissan Leaf (and I think the Volt) measures efficiency in miles per kWh. This may be easier to understand if you are used to thinking in miles per gallon (or km/L) because you know the capacity of the battery (fuel tank) and how much fuel it takes you to go a mile. For m/kWh the higher the number the better, just like mpg. The conversion is m/kWh = 1 / ( Wh/m) * 1000. So a "normal" 300 Wh/mile is equivalent to 3.33 m/kWh.
     
  8. Vrrooom

    Vrrooom Member

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    This is a great method, but it is important to know that a 85KWh car does not have 85KWh available for driving. So a full tank would be closer to 78 KWh.
     
  9. mikeash

    mikeash Active Member

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    Just to give some fundamental definitions to complement the explanations:
    • One joule is equal to the energy involved in pushing with a force of one newton over a distance of one meter. In terms of heat, it's the energy needed to raise the temperature of one gram of water by approximately 0.24 degrees Celsius.
    • One watt (W) is equal to one joule of energy consumed per second. A 100W light bulb consumes 100 joules of energy per second, enough to heat 100 grams of water by 0.24 degrees per second.
    • One kilowatt (kW) is just a thousand watts. Kilo means thousand, as I'm sure you already know. A kilowatt will run 10 100W light bulbs.
    • Here's where it starts to get confusing: a kilowatt-hour (kWh) is the amount of energy you use if you use one kilowatt for one hour. It's the energy consumed by running a 100W light bulb for ten hours. A kilowatt is 1000 joules per second, and an hour is 3600 seconds, so a kilowatt-hour is equal to 3.6 million joules.
    • Finally, a watt-hour (Wh) is the energy of one watt for one hour, or 3600 joules. It's useful when talking about smaller quantities of energy.
    To relate this back to stuff about your car:
    • The battery stores energy, usually measured in kWh. The model numbers correspond (roughly) to battery capacity in kWh.
    • Driving uses energy for each mile. The amount of energy you use per mile says how efficiently you're driving. Since the quantities are lower, this is usually given in watt-hour per mile, or Wh/mile. Typical numbers are around 300Wh/mile.
    • You can divide your battery capacity by Wh/mile to get approximate range. For example, divide 85kWh by 300Wh/mile to get 283 miles of range, slightly more than the official 265 miles of range for my 85. (The discrepancy is because it doesn't have 85kWh usable, some is held in reserve or is marketing fiction, not sure which.)
    • Charging provides a certain amount of energy over time, and is usually measured in kilowatts. A typical 240V/40A L2 charging setup provides about 10kW. Divide this into the size of your battery to get the approximate 0-100% charge time: my 85kWh battery takes about 8.5 hours to charge at 10kW. I might see 120kW at a Supercharger until I hit the taper at around 40%, going from 0-40% involves adding 85*.4 = 34kWh of energy, divide by 120kW and we can see that this will take about 0.28 hours or 17 minutes.
     
  10. Soundchasr

    Soundchasr Member

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    Thanks everyone. Need to review and.... digest. :)
     
  11. Fusion

    Fusion Member

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    What's a good mile/kwh for the Tesla S? I use to get 3.2 mile to 1 kwh on my rav4 ev.
     
  12. mikeash

    mikeash Active Member

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    Official rated range is based on roughly 300Wh/mile, or 3.3 miles/Wh. That's not too hard to achieve, although of course cold weather or going 75+MPH will hurt.
     
  13. wdolson

    wdolson Active Member

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    I'm going up and down hills all the time which impacts my efficiency on both the last leg and first leg (to get to our house you have to go almost to the peak of a hill, then back down again, so there is hill climbing both ways). I usually average between 320 and 340 Wh/mi around town.

    Hard acceleration runs down the efficiency a lot. My SO manages to hit 1 KW accelerating the car one time. I don't see why anyone would want or need a P90DL unless they wanted to race it. The 90D has plenty of power and torque for any daily driving situation.
     
  14. st50maint

    st50maint Member

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    Why are the scales of the 5 mile, 15 mile, and 30 mile graphs apparently different?
    (I asked about this once before and the one answer mentioned averaging but I still do not completely understand.)

    On the 5 mile graph the peaks are off the scale.
    On the 0 to 5 mile portion of the 15 mile graph the peaks are not even close to those numbers.
    Since the vertical scale is apparently the same (-300 to +900) I assumed the waveform would be similar.
    What am I missing?

    [​IMG] [​IMG]
     
  15. SageBrush

    SageBrush Active Member

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    Amusing :)

    WAG: the car is calculating Wh/time and then taking a distance input for the graph, but the Wh/time polling interval is longer if the more condensed graph is requested. Either that, or the car is plotting a rolling average to smooth out the graph.
     

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