EKM Digital Submeter Measures EV Charging Efficiency

Electric vehicle owners may now know that 10-12% of drawn electricity is lost during the charging process when AC is converted to DC.

Energy usedThe “Since Last Charge” indicator on the Tesla Model S driver’s console keeps count of distance, energy used and average energy used per mile since the last time you unplugged, but matching this energy consumption to what you’re being billed by your company is more difficult than one might think.

Taking the 10.6kWh Total Energy readout from the dash and multiplying it by my billable electricity rate of $0.1670/kWh, I would think that it costs me $1.77 to drive 35.3 miles. However the actual costs are slightly higher because of the power conversion losses when charging. Charging via a standard 110V wall plug in the US is reported to be much less efficient than a NEMA 14-50 hence you’ll be using even more energy to reach the same state of charge.

Tesla’s charging calculator appears to take into account some of these charging loses. According to the calculator, 35 miles of range added to the Model S requires 11.6kWh of energy although I was able to achieve that with through 10.6 kWh as seen through my dash readout. This could imply a 91% charge efficiency but of course this is a very loose calculation and based on many assumptions such as the speed in which you drive, elevation changes and weather. It’s not simple to calculate how much actual energy you’re using to charge your Tesla Model S.

The EKM Digital Submeter

EKM Meter InstalledThe solution I found was to put a submeter between the EV charge outlet and main power supply. There are many types of meters on the market from basic kWh counters to advanced meters that can broadcast actual use over a wifi network, plot graphs, etc. After doing some extensive research, I decided to go with a metering solution from EKM Metering. My electrician did some independent research and came up with the same brand so I felt pretty good about my choice. I ended up purchasing the 100A kWh EKM digital submeter and enclosure.

My electrician was able to complete the installation in 3 hours with a portion of the time spent on retrofitting the EKM enclosure to accommodate for the larger 240v conduit. Total cost spent for products, labor and materials was $292.

How Does it Work?

The EKM submeter is like an odometer but with an energy readout that continuously increments as power is being drawn. There is no reset button. A blinking red light indicates that 1.25Wh is drawn per single blink of the LED.

Meter Reading

Measuring actual energy consumed over a specific time period requires taking some notes. Here are the the measurements and results of my tests:

Model S Charging Efficiency

My results indicate that there’s approximately a 85% charging efficiency for my Tesla Model S which is less than the 91% efficiency that Tesla Motors seems to be using in their online calculator.

I’ll be performing several more tests over a longer period of time. I want to look at power draw differences while the Model S is asleep and also analyze variations in energy consumption due to software updates.


Overall the EKM Digital Submeter is a nice addition to any EV charging setup if you’re looking for a true picture of how much energy usage is going into your Tesla Model S / electrical vehicle. Assuming a $0.167/kWh electricity cost and 325 Wh/mile, the cost of the meter plus installation would require 5,380 miles of EV driving to break even.

If you’re looking for a cheaper alternative, simply add 15% to the energy usage readout on your EV to approximate the true energy cost per EV mile driven. Drive clean and drive smart!


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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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  • liuping

    Were you changeling a the full 40Amps?

    I know some people dial it back to 30Amps since they feel it will be easier on the battery. Charging as 30Amps will be slightly less efficient than charging at 40Amps.

    • Yes, full charge at 40A 230V on my NEMA 14-50. I haven’t had any advice from Tesla on dialing it back. Not sure that advice would make sense since they encourage the HPWC which is 80A all the time (unless you also dial that back).

      • liuping

        I agree, I don’t dial back, but I’ve seen many people mention it on TMC.

  • drkennethnoisewater

    Does the meter itself affect efficiency?

    • The meter says its accurate to within 1% and it draws less than 1W. For small draws that could be significant but when we’re dealing with 10’s or 100’s of kWh that 1W is going to be negligible. The study will be more interesting over a longer period and this was more of an initial experience and install versus a longer term real data summary.

  • dp

    Although it is still a matter of energy efficiency, it appears there are two additional power drains that are not captured by the trip energy counters in the car: power consumed while the car is off, and power consumed to condition the battery from cold. I have found that if I carefully drive 5 miles at a certain wh/mi first thing in the morning and measure the range decrease and trip power, and then do the same thing for the next five miles (with a warm battery, same Wh/mi), the trip energy used is the same, but the change in range is different over the second 5-mile segment. The delta must be the battery warm-up energy which is not being tracked by the trip energy counter. Much has been said about the vampire loss, so no need to comment on it here. In other words, by calculating your charging efficiency using the trip counters, you are including multiple efficiency drains, not just the charging efficiency drain. I would be interested to hear the results of changing your charging units from miles to kWh on the car display and comparing that number to the sub-meter.

    • I think there are a lot of things from charging efficiency to reporting to vampire loss etc that can happen once power leaves the outlet. This sort of meter, while expensive, clears up any confusion as to the actual cost to drive a mile on an EV.

      • dp


  • Peter Murray

    Inverter efficiency seems to also be dependent on if you are charging close to the inverters capacity. I have solar inverters and their peak efficiency is when they are running in the mid 90% of capacity.

    So if you have a single inverter your most efficient charge rate in theory would 40amps. If you have twin chargers and they load both inverters in parallel 80amps.

    How did you run your test and do you have single or dual chargers?

    • I have dual inverters, so i’m capable of a total of 80A charging. My NEMA 14-50 only supplies 40A so I charge at 40A.

      The test was simple, record the kWh meter reading, charge up, record kWh meter reading, drive, charge back up and record kWh meter reading. Subject to law of small numbers and measuring over a short period. Will do a longer term study as time passes.

  • Top 12

    Just install some solarpanels like 30 x 250 watts and then forget about this matter for ever. That is what I did some years ago and everything works just great.