Charging a Tesla Model S Might Be Costing More Than You Think

Reported Energy UsedWhen you fill up a normal ICE car you know exactly what your costs are for the fuel. With an electric vehicle it is not that simple. There is a charging efficiency factor that comes into play which means that the reported amount of energy used could be understated and lower than the actual energy used. In a previous post I wrote about installing an EKM Digital Submeter on my NEMA 14-50 outlet to measure actual power usage of the Model S against the reported power displayed on the driver’s digital dash display. A month later and armed with more data, the energy loss I’m seeing is larger than I had originally expected.

Test Setup

I charge at home 99% of the time and in the last three months I’ve logged 7,500+ miles driven, one trip to the Supercharger and two visits to the Tesla store’s High Power Wall Connector. I have a professionally installed NEMA 14-50 outlet at home. I’m using the factory supplied Universal Mobile Connector (UMC) as the cable between the outlet and the car. An EKM digital sub meter measures actual draw from the outlet and is accurate to within 1% and does not add any measurable load of its own.

Methodology

On the “anniversary date” of taking delivery of my Model S I recorded all of the pertinent info that was displayed before resetting the Trip A meter. Before driving the next day I record the reading on the EKM meter. That way I’ve got the mileage and the Tesla reported power usage over the period driven. This process will let me see a bunch of information that I plan on tracking over time, as follows:

  • Monthly miles driven
  • Monthly kWh used as reported by the Model S
  • Monthly kWh used as reported by the EKM meter
  • Monthly Average energy used

I plan on using this information to look at how average energy used changes as the months/temperature changes and perhaps as the Model S gets more miles on it. While I don’t drive consistently on any given day (test drives, special trips and the like), the numbers will average out and my driving style is not likely to change much after 30 years of driving (yeah I’m getting old but the Model S makes me feel young again!). I also drive pretty consistent patterns of commuting with a lot of miles to the same places which helps average out the special trips to locations with different terrain/conditions. Basically, while the conditions aren’t perfectly stable over time, the averages and data from this real world testing will be pretty accurate.

The Data

The last period (6/21 – 7/21) was my first full period with both the car and the EKM meter. A month of driving and charging, especially with the miles and kWh’s involved is a decent period over which to look at the results versus the 2 days from my prior blog post. Here’s the data: Actual Energy June-Jul In the above table you can see that the Model S reported 728 kWh used during the period but the meter reported 894 kWh used. This means my charging efficiency is only about 82% and electric usage (and cost) is 23% higher than I may have expected based on the readings the Model S provides. For that month this is an extra $26 of charging cost which is a small number but a notable percentage of the total. The good news is that even using this larger kWh number, the savings versus driving my old ICE car for energy alone comes in at $334 — I’m saving $334/month in gas driving my Model S!

Summary

Research suggests that an average charging efficiency loss ranges between 10-12%. Over this one month period of over 2,400 miles I’m seeing an 23% loss using the standard home charging setup that Tesla recommends. Many people quote an 85% charge efficiency for Tesla, and Tesla’s own charging calculator appears to assume a 91% charging efficiency which is quite different than the 82% actual charge efficiency I’ve measured and  significantly worse than the average industry charging efficiency. It would be great to see another Model S owner do a similar test using the HPWC setup at home and see if they get results similar to what Tesla is providing. I’d love to do the test but I’m not quite ready to shell out $1,200 plus electrician costs to get that data — assuming a cost of about $3,000 all in it would take me over 20 years to break even assuming the HPWC improves my efficiency by 10%. From the results above, my conclusion is that the Model S charging efficiency using the standard home setup is 5-10% worse than other EVs on the market.

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Rob M.

Author: Rob M.

Rob's passion is technology and gadgets. An engineer by profession and an executive and founder at several high tech startups Rob has a unique view on technology and some strong opinions. When he's not writing about Tesla, Solar City and related news he's off hacking code, playing video games or hiking the trails of New England with his dog.Rob's Model S details: S85 | Grey / Tan Interior | Obeche Matte Trim | 19" standard | Parking Sensors | Sub-zero | Pano | Premium Lighting | Dual Chargers | VIN: 36801 | Took delivery 4/21/2014.Feel free to follow him on twitter @teslaliving for random thoughts, lots of pictures, news updates etc.

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  • mgboyes

    A couple of UK owners have had calibrated meters installed for their chargepoints.

    They have much less data than you (after only a month of ownership) but are reporting that around 80% of the power delivered to the vehicle ends up adding to the battery’s estimated level of charge, which matches your figures.

    But I don’t think this data can lead you to say that charging is only 80% efficient, because not all the power that goes into the car goes into increasing the battery’s charge. Some is used to run the climate control and other systems if the car is powered on, and some is used to counteract charge losses when the car is sitting for a long time.

    I guess it becomes a bit of a semantic debate about what factors “charging efficiency” should cover…

    • http://teslaliving.wordpress.com/ Teslaliving

      True, some of it goes into stuff that doesnt move the car, but it is the cost of energy involved in the ownership. Just like you burn gas to power your A/C in an ICE car etc.

      • mgboyes

        Oh I completely agree that the fair comparison for “liters at the pump” has to be “kWh at the outlet”, I’m just not sure it’s fair to say that a Model S has 80% charging efficiency that’s all.

      • http://teslaliving.wordpress.com/ Teslaliving

        True, its more about whats reported at the wall vs what the car reports its used. That gap is 20%.

  • mgboyes

    Also I don’t know whether the trip gauges actually measure total electrical energy flowing out of the battery, or just the energy usage that is associated with a journey. If you charge your car, disconnect it, leave it for a week, then get in and check the trip meters does the “total energy since last charge” figure read 0kWh or something else? If you stop during a journey and sit in the car in P with the aircon and systems all running does the Avg Energy figure increase over time or stay the same?

    • http://teslaliving.wordpress.com/ Teslaliving

      From everything i’ve read it only counts energy when the car is powered on. It will count it if you’re on and stopped (at a light, for a passenger etc) but if it uses energy when not powered on (vampire drain for example) it does not track that.

  • John Tamplin

    I measured it at 86% efficiency (using a TED5000 in my garage subpanel, and comparing data reported from the car via the streaming API), but I generally charge at 55A and only to 90% almost all the time. It definitely becomes less efficient as you do a max-range charge (you get about half of the energy into the battery, and the rest is waste heat and running the heat pump to remove that heat) – also, if you charge higher, the resistive loss in the cabling, inverter, and battery goes up with the square of the current so doubling current quadruples energy lost to resistive heating.

    • http://teslaliving.wordpress.com/ Teslaliving

      Interesting. I’m only able to charge at 40A on my NEMA 14-50 so perhaps thats the difference? Are you using a HPWC?

      • John Tamplin

        I am using an HPWC, but any EVSE is mostly a glorified extension cord with a contactor and ground-fault checks in the AC circuit, and then a microprocessor to communicate with the car. I doubt any of those differences matter, though it does use thicker wire to handle the heavier current, which has slightly lower resistance. However, I also measured a LEAF in the same garage connected via #6 wire (probably the same as you have for your 14-50) and found it got 87% efficiency (slower charge rates probably explains that difference). If you have a longer run to your breaker box that might explain it as well. I assume you rarely max-range charge? What voltage do you normally get at the car?

        My guess is the difference is the accuracy of our two meters – don’t know which is more accurate (though my numbers did match up exactly against what Nissan claimed).

      • http://teslaliving.wordpress.com/ Teslaliving

        Yeah I have a long run from the outlet to the main panel (and its off a sub-panel). The meter is between the sub-panel and the car though. I get about 230V, 40A.

    • Alex Waardenburg

      John, You are right that increasing current would drastically increase the inefficiency due to resistance, but I wonder if that is significant compared to the loss of efficiency that comes from running the heat pump to keep the battery cool during charging. Depending on how the heat pump runs this *might* mean that it would be more efficient to charge it more quickly. The heat pump is a significant load, isn’t it? Could this explain the discrepancy?

      • John Tamplin

        For me, it only runs intermittently when charging, but I haven’t charged at 80A enough to see how much cooling is needed when charging that fast. However, I don’t think it is that much, since after using the Supercharger the HVAC was drawing between 1-2kW cooling the battery, and that generates a lot more heat than the HPWC.

  • Brian

    There are many factors not accounted for that go into what he is “saving”. First off you have no idea how hard the Tesla is being driven. My experience shows people who drive electric cars and hybrids alter their driving habits to maximize efficiency while people in ICE cars generally make no effort to hyper mile. Based on the figures his “old ICE car” was getting 18mpg which is well below average for a modern passenger car. If you trade a mid size SUV for a Tesla yes you can save on fuel costs. A Mercedes Benz E250 BlueTEC costs $18,000 less than the Tesla. It also gets 33 combined MPG which is probably a conservative figure given that most diesel cars out perform EPA ratings. The Mercedes fuel cost for 2417 miles would be roughly $282 (based on 33mpg and $3.85/gal). In that case the Tesla would have saved $139 in monthly fuel costs. The break even for the extra $18K invested in the Tesla is almost 11 years. He also didn’t mention how much the installation of the charging station costs.

    • TedKidd

      Interesting article! Guess these cars have phantom load! I think the kwh per mile measured probably doesn’t include these losses, so if your car sits idle for long periods you have some lost electrons.

      My tdi sportwagen has a fuel cost of 11¢ per mile for 30k miles. My aunt and uncle’s Tesla in Jackson costs about 2¢ a mile with sub 10¢ electricity, standing losses or no. http://bit.ly/tetontesla

      Brian, have you driven a Tesla? Not sure it’s all that easy to drive like a hypermiler unless you are short range to the next supercharger. (And at that cost there isn’t much to be gained unless you enjoy driving erratically or annoying drivers around you.)

      I had an e320cdi. Also a hard car to drive gently. Not sure where you buy diesel, but if you pay 16¢ per kwh, I bet diesel is closer to $4.50.

    • http://teslaliving.wordpress.com/ Teslaliving

      Old ICE car was a 2007 Acura MDX. I got an average of 20MPG (super/premium Gas)

      I compared the Model S with getting a 2014 MDX here:

      http://teslaliving.wordpress.com/2014/03/03/cost-justification-a-tie/

      The savings i’m calculating is against a NEW 2014 MDX.

      My wife has a ML350 BlueTEC. It gets good fuel efficiency (diesel) at about 28MPG in real world use for what we do which is not far off what the new MDX does. Ongoing maintenance is really expensive on the ML and so is diesel. Over 7 years the comparison will be about the same as the MDX one I did.

      Charging station for me was $675:
      http://teslaliving.wordpress.com/2014/04/19/delivery-preparations/

  • No way

    “In the above table you can see that the Model S reported 728 kWh used during the period but the meter reported 894 kWh used. This means my charging efficiency is only about 82% and electric usage (and cost) is 18% higher than I may have expected based on the readings the Model S provides.”

    The cost and the kWh used compared to the number given by the Model S is 23% higher, not 18%. It’s basic elementary math :)

    • http://teslaliving.wordpress.com/ Teslaliving

      Nice catch, you’re right.

  • In The Shadows

    The assessment of percentage is wrong, because the EKM meter wasn’t use across the board,But you assume their information was correct without any tests,you used it without any validation, so it can only be use against Tesla, claims on how much energy it used!

  • ka9q

    Charging speed is almost certainly a major factor here. Every rechargeable battery must be charged at a higher voltage than the voltage at which it discharges, and the only way to increase charging speed is to increase the charging voltage (subject to a strict upper safety limit, especially for Li-ion batteries). Thermodynamics requires this to drive the chemical reactions. It also says that the faster you discharge the battery, the lower the terminal voltage will be. Assuming the Coulombic (amp-hour) efficiency is 100%, a good approximation for Li-ion batteries since they can’t electrolyze water, the energy efficiency of the battery itself is equal to the discharge voltage divided by the charge voltage.

    Moral: charge slowly, discharge slowly. Charging should be at the lowest rate that will give you the state of charge you want when you next need the car (or when the TOU rates go up). No need to charge at 50 kW for a half hour after midnight when you’ve got until morning.