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Roadster Energy Reporting and Efficiency

Discussion in 'Roadster: Technical' started by tomsax, Dec 13, 2009.

  1. tomsax

    tomsax Member

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    #1 tomsax, Dec 13, 2009
    Last edited: Dec 13, 2009
    I just posted some information on my blog about the Roadster's energy efficiency computed by measuring energy put into the car from charging with an electric meter divided by miles driven and how this compares to the energy and efficiency numbers reported on the Roadster's screens. Here's what I've learned...

    For the month of November, I drove the Roadster 762.2 miles. That's mostly with just me in the car driving a variety of city and highway miles. I tend to drive enthusiastically most of the time, but the month also included a roundtrip drive to Longview, WA on cruise control at 55 mph.

    During the month, I put about 247.8 kWh into the car from the wall (213.3 kWh metered from my garage plus approximately 34.5 kWh from an unmetered NEMA 14-50 outlet in Longview). That's 325.1 Wh/mi and includes charging losses, battery pack self discharge, heater, headlights, etc. That's my wall-to-wheel number and is based completely on things I can measure.

    From July 25th to August 27th, I drove the Roadster 696 miles and pulled 234 kilowatt hours (kWh) from the grid, giving us 336 Wh/mi. That included some hot weather and four 1/4 mile runs at Pacific Raceways.

    On individual charges, I see efficiency vary from 240 Wh/mi to over 400 Wh/mi, and obviously much higher for things like drag racing.

    I charge consistently at 240V and 40A at home. In Longview it was 230V and 40A. Because of charging overhead, I assume I would get slightly better charging efficiency if I charged at home at 70A. So, my numbers are just that, my numbers. Another driver would get different numbers depending on driving, weather, road conditions, and charging habits.

    The EPA estimates documented in the paperwork for our car say 260 Wh/mi city and 290 Wh/mi highway. I've seen information from early 2008 Roadsters that had the EPA numbers and 340 and 360 Wh/mi.

    You may have heard Roadster owners talk about numbers well below my 330 Wh/mi numbers. These are most often the number reported by the car's info screen which are not wall-to-wheel numbers, and in fact are (as far as I know) not at all documented as to what that number means. I have figured out some things about the numbers reported by the car, which I'll now explain.

    For the month of November, the Roadster's trip meter says that I used 207.9 kWh, and thus 272.8 Wh/mi. But what does that mean? Did I push 207.9 kWh into the motor, or is that net of energy pushed back into the pack from regenerative braking (regen)? Does it include energy used to run the accessories and/or running the coolant pump and fans during charging?

    On the "Energy History" screen, the Roadster tells me my "net energy used" for the month was 233 kWh and that I got 26 kWh from regen. What does "net" mean? I would assume that "net" means "net of regen," i.e., power from battery pack minus power into battery pack from regen. Except, if I compare those numbers to what the trip meter says, I notice that 233 - 26 = 207, which is suspiciously close to the energy use number reported on the trip meter.

    From that, I infer that the trip meter's number is net energy use from the battery pack (power drawn minus regen put back in), and thus the so-called "net energy" from the energy use screen is really the gross energy pulled from the battery pack including energy that went into the pack from both wall charging and regen charging.

    Do these numbers include the energy spent on accessories? Is the difference between what I put in through charging (247.8 kWh) and the car's reported net energy use (207.9 kWh) just charging losses or does that also include accessory use? I have no idea.

    The only number I can stand behind, and the only number I can compare with other electric vehicles, is the wall-to-wheel number. The efficiency number reported on various of the Roadster's info screens is useful for understanding how driving style and conditions affect efficiency and for predicting/optimizing range, but is seemingly useless in any other context.

    I believe the same is true of any efficiency number for the Leaf given out by Nissan, or any other EV manufacturer or driver, unless that number is as clearly defined and directly measured as the wall-to-wheel number.

    It used to be that the Tesla screen reported an energy number after each charge that was much lower that what was actually drawn from the wall. I suspect that was the energy that actually made it into the battery pack, but I never saw it defined by Tesla. More recent firmware versions are reporting a number that is close to the number I read from the wall meter (and averaging multiple consecutive readings together agrees to within 1% of the wall reading). This is a big step forward for drivers who want to monitor their actual wall-to-wheel energy use and efficiency, but don't want to go to the expense of installing a dedicated meter. It would be a real benefit to the Tesla community if Tesla would (a) define the number they currently report and (b) make the energy drawn from the wall across multiple charges easily available.

    Regarding range on a single charge, my personal record is 192 miles driven with a passenger in 100+ degree weather starting with a bit less than a full charge and ending with 10 miles of range left. On the trip back from Longview in cool weather, I drove 136.9 miles using cruise control at 55 mph using 55% of the battery. To the extent that you can extrapolate that to the full battery, that figures out to about 249 miles of range. On the trip down to Longview earlier the same day, also using cruise control at 55 mph, it was raining and colder, so I had the wipers, headlights and heater on and used 65% of the battery pack, for an extrapolated range of 208 miles.

    My car is a 2008 Tesla Roadster with firmware version "3.4.15 15" (upgraded from "3.4.13 15" on 11/15/2009).
     
  2. Ben W

    Ben W P85 #61, Roadster #108

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    Tom,

    Interesting numbers. Be sure to take into account the relative charge level of the Tesla's battery at the beginning and end of the month. I wonder if there is any significant change in real-world efficiency between the 2008/2010 Roadster, and Roadster Sport?
     
  3. tomsax

    tomsax Member

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    Ben,

    The numbers quoted are all full-charge to full-charge (in the same charging mode) to eliminate any differences there. I'm now using a custom program to do all of my logging and I will teach it to deal with small charging differences, but I'll still prefer to quote full-to-full numbers.

    I agree it would be interesting to know about any efficiency differences between models. The logging we did for the group drive to the Wild Horse wind farm indicated that if there is a difference, it isn't large.

    I hope to make my app available in the next few months, so maybe we can use that as a way to compare data between vehicles.
     
  4. Adm

    Adm Active Member

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    I have reserved a Model S and I am trying to estimate my yearly energy consumption driving this car.

    Apart from this thread I haven't been able to find numbers from Roadster owners. I know I can't translate these numbers 1:1 to the Model S, but I would have place to start.
     
  5. LST

    LST Senior Member

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    After driving my Roadster for nearly a year my average consumption was around 170 Wh/km. More in winter, less in summer. I've been driving 8'000km as of now, happily, hassle free. I would guess, the Model S would consume more (weight).
     
  6. tomsax

    tomsax Member

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    Is that the number reported by the Roadster? The Roadster reports energy use from the battery to the wheels and doesn't include the energy lost during charging and between drives.

    Anyone interested in figuring out what their energy use will be will need to get wall-to-wheel numbers, like the numbers I reported a few posts back in this thread.

    The number reported by the Roadster is more suited for projecting range and for comparing driving efficiency between drivers.
     
  7. ChargeIt!

    ChargeIt! Member

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    #7 ChargeIt!, Aug 10, 2010
    Last edited: Aug 10, 2010
    In answer to Adm's question: To make it as simple as possible ... at the risk of losing some accuracy (but that's ok, since the Model S will be different anyway) ... from Tom's data the magic estimated number is 3mpk. That's 3 miles per kWh for the Roadster wall-to-wheel (333Wh/mile). Tom should chime in here to confirm, but that's what it looks like from his data and everything I have read and heard from other sources also.

    So ... to get a STARTING estimate of energy cost for Model S: take the annual miles you drive, divide by 3, multiply by your electricity cost (in $/kWh). For us ( if we had a Model S today ): ( 18,000 miles-per-year / 3 mpk ) * ( 0.17 $/kWh ) = $ 1,020 per year. ( $85/month ). (Wish we were paying Washington State's 6 cents per kWh !)

    (This is for temperate climates with some A/C and a little heat.)

    Edit: Just a thought ... 18,000 miles per year. Using 6 MEGA-WATT-HOURS ! Amazing ... thinking about it that way. Of course, the rest of the house uses that much or even more every year, but we don't really think about it until now when we start computing at this level of detail because of EVs :smile:
     
  8. LST

    LST Senior Member

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    Hi Tom

    I knew this question would be coming ; )) The numbers are (of course) wall to wheel. I have a meter installed solely to monitor the 32 amp socket. I drive approx. 50 % town, 50 % highway. I never use AC, in winter of course i heat. I always have the lights on.
    BTW : The roadster's reading does not differ so much from what i see on the meter.
     
  9. Adm

    Adm Active Member

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    The numbers don't differ that much. Translating Tom's number to the metric system it's 208 Wh/km vs LST's 170 Wh/km. That difference could be explained by the absence of trips to the dragstrip.
     
  10. tomsax

    tomsax Member

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    ChargeIt! - You are interpreting my values correctly and that's how I would do the annual electrical cost calculation.

    I get wall-to-wheel values between 320 and 330 Wh/mile pretty consistently except on rare track days. I generally drive with a pretty light foot, so I'm impressed with LST's numbers which are equivalent to 274 Wh/mile.
     
  11. Tomihly

    Tomihly Rdstr #350; S #13833

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    #11 Tomihly, Oct 14, 2010
    Last edited: Oct 14, 2010
    I have had my roadster 2.5 for a bit more than 2 weeks now.
    Here's some data of my first longer return trip, which I intend to undertake every one to two weeks from now on (the data is read from the screens inside the car, i.e. not wall to wheel, since I have no metered outlets yet): total trip: 555km (roughly 275 km each way); total net energy use (i.e. including regen, which on average seems to reduce the total energy value by about 6-8%): 93.22 kWh; Trip duration: 7:00hrs (-->average speed 80km/hr); roughly 80% highway @ speeds between 80 and 120 km/hr, the rest @ 60-80 km/hr; the last(outbound)/first(inbound) 14 km 1100m uphill/downhill.
    I charged the battery in "range"-mode at the beginning and at the far end of the trip. Km's left: about 30 km (approx. 10% battery charge) at the far end, and about 55 km (approx. 15% battery charge) at the end of the trip.
     

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