Last week was the 4th anniversary of the activation of my home charging set-up at my condo, so I thought it would be interesting to see how much energy I had purchased from our local utility, Eversource. (I live in eastern Massachusetts.)
I charge the car (a 2015 70D) with a Tesla Wall Connector connected to a 240 Volts single-phase 50-amp circuit. The cable run from the meter to the Wall Connector is roughly 25 feet. I usually charge at 40 amps.
There is a separate service and electric meter for charging my car, so it is (almost) easy to see how much energy the car consumed. I say “almost” because there was a brief gap in my data for which I made an adjustment. Plus, I have a motion-activated security lamp on the same service. The lamp does not run for very long when it activates, so I decided to ignore the lamp’s energy use, as it would be very small compared to the car’s.
In those four years, the circuit consumed 17,827 kWh, while the car went 41,033 miles. So, based upon these raw numbers, we could say that the car charging (including charging losses plus precooling and preheating) consumed an average of 348.7 kWh/mile driven. That compares to the lifetime average consumption shown by the car in July 2019 as 326 Wh/mile. (I lost my historical data at that time because of a computer repair.)
Unfortunately, that is an incomplete picture, because it does not allow for charging elsewhere, such as use of Superchargers and destination chargers on trips. I do not have very good records of charging away from home. So, I made an estimate of the mileage driven away from home over those four years and decided to allow 1,000 miles per year as a rough but reasonable allowance. So that means the Eversource energy was used for about 37,033 miles. The average energy use then looks like 481 Wh/mile, a much higher figure, and a bit startling. I know that charging losses have been estimated at 10% or so, but this would be quite a bit higher.
Most likely, the largest source of extra consumption is preheating and precooling the car. I tend to use that function quite a lot, because my car is parked outside, winter and summer. The car does not consider the energy used for that in the displayed energy consumption, when the car is connected to shore power. Since the car's heater can operate at as much as 6 kW, prolonged use of preheating in winter can add considerably to total energy consumption. Air conditioning does not consume as much, but even that does add something to energy costs.
At my current average price per kWh of about 23 cents, that translates to approximately 11 cents per mile for energy. That is not much if any savings compared to my previous car, which was a Mercedes that got about 20-23 mpg on premium fuel. Which I why I tell people around here that they should not buy a Tesla, or any other EV, with the goal of saving on energy. (The exception might be if they happen to live in a town or city with a municipal electric department, which typically provides energy at a much lower cost.)
My numbers do contain some assumptions, the most important of which is my allowance of 1000 miles per year for travel charged elsewhere. That may be conservative, and I hope it is, but it is not too far off. In any case, it is worthwhile to realize that charging losses may add to the energy consumption reported by the car. And in my case, preheating and precooling probably contribute substantially to the actual total energy used and the cost of it.
One other point: The precooling and preheating would be a smaller factor in cost per mile if I drove more. But I am retired, so I do not commute. And of course, since mid-March we have been driving very little because of the Covid-19 shutdown.
