When we installed the NEMA1430 receptacle in our garage subpanel to charge our Tesla Model X, we also installed a digital kilowatt hour meter (kHr). See the video on my install here. My intent was to determine the efficiency of our charging. I wanted to know much electricity we were buying from the power company and how much electricity was being put into the car's battery. During charging, some electricity is lost to heat; therefore, the efficiency is how close the power out of the wall equals the power into the car. What I found, or thought I found, was that our efficiency was and incredibly lower number around 65%. I had expected a number in the 85% range. So, I set out to determine where the difference came from. Was the digital meter I was using inaccurate? Was charging on a 30amp circuit less efficient then on a 50amp circuit.Charging on a higher amp circuit is typically considered more efficient. I never considered that the numbers coming out of the Model X were inaccurate.
So, to study the lower efficiency, I set out to do several debugging steps. First, I ordered and installed a real analog kilowatt hour meter next to my digital meter. I bought this meter off eBay. As power companies around the country replaced their analog meters with digital meters, the surplus analog meters became available for purchase. This meter was calibrated and certified as accurate plus a 2 year guarantee. I built a metal enclosure for the meter and put a NEMA1430 and NEMA1450 receptacle in the box. What I found after 5 days of charging with both meters active is that the two meters have VERY close readings. The digital meter read 161kWh, and the analog meter read 158.7kWhs. The difference is 2.3kWhs. The digital meter level of accuracy was only good to +- 1 kWh, but after number of days, that value became less of an issue. The longer the two meters ran, the less that mattered.
I went into that test expecting the digital meter to be accurate. I've used them before with good outcomes. I was not surprised to validate that it was as accurate as a real analog meter.
I also ran a test where I used our car dashcam and a thermometer to monitor the car charging one night. I wanted to see how many times the battery fans came on and whether or not the garage air condition which is 80000 BTU could keep the garage at the 80 degrees that it is set to. I found that the fan came on just a few times and that the air condition had no trouble keeping up. So, the problem also didn't seem to be an abundance of lost heat.
This encouraged me to research more on the Tesla forums. What I found is that the Trip output which I had considered as the golden value for the amount of kWhs used by the car really did not include the entire amount of energy that the car consumed. That is, the Trip output o.nly includes the kWhs used by the car when it is in or driving. So, if you are parked and running the AC or cabin overheat protections cycles the AC on and off all day or cabin preconditioning turns on the AC to be ready for you...all of those electricity burning activities are not counted. What I found is that there is no total energy consumption value that is available on any of the LCD screens in the car.
So, the math that I had been doing every week to calculate the charging efficiency was wrong due to my assumption about the Trip meter.
Still wanting to understand our charging efficiency, I looked into a website called http://www.teslafi.com. This website uses the Tesla API to extract data from the car. It includes a lot of interesting data like where the car drove, how fast it was going, and of particular interest to me, the amount of energy used. It even does the math to calculate a charging efficiency. I pleased to discover that this app thinks our charging efficiency is 86% which is right in the ballpark with what I expected to begin with.
This website offers a free trial and then is $5 a month or $50 for a year. Right now I am doing the trial but am intrigued by all the wonderful geeky information and will probably sign up for a year when the trial ends. Check out the following YouTube video which provides an overview of the app and a promo code for an additional two weeks of trial.
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