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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