Electricity Cost and the Model S

Post Updated December 26, 2013

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We’ve lived with and have been driving gas powered cars our whole life. Nothing is more iconic than a car, or more advertised on television. So understanding how a car uses gasoline in the course of driving and the economics around purchasing and using the fuel are pretty well established in our minds. We don’t even think about it.

  • We know or can readily find out how much gas costs per gallon (a lot)
  • We can easily recognize a gas station on the side of road (usually one on each corner)
  • Although there are choices on which gas to choose, most people don’t agonize over which button to select
  • Using the pumps and paying is fairly straightforward, except if you live in New Jersey and Oregon (State laws prohibit those citizens from operating fuel pumps)
  • We pretty much know the quantity of gas our tank holds and how many miles per gallon (MPG) we can expect

Many people don’t know their exact MPG, but most know a ballpark number if asked. When driving the dashboard has a fuel gauge so we know when we’re getting low. Many cars now provide real time MPG tracking feedback messages and readouts to help you know how your driving behavior impacts amount of gas burned. We get it.

images-2How an electric vehicle (EV) stores uses and recharges electricity is less well understood. Let’s take a step back and cover some of the basics of this power source. Miles per gallon is obviously not relevant, so the Environmental Protection Agency developed a Miles Per Gallon Equivalent (MPGe). This is a measure of the average distance traveled per unit of energy consumed. The EPA simulated five different driving conditions and arrived at the following statement. 33.7 kilowatt hours of electricity is equivalent to one gallon of gasoline. A kilowatt is a unit of energy equal to 1,000 watt-hours. Our home operates on kilowatts and we see that familiar measure on our monthly bill. We now have a basic builidng block that we can map to a gallon of gas.

Kw1

When you’re driving an EV it’s all about Watt Hours Per Mile (Wh/mi). The Model S has a battery life indicator in the center of the dash that constantly displays how many miles you have left in your battery from a rated range calculation. As with a gas car there are many factors that impact efficienty. Speed, quick starts, temperature, how many things you have on in the car, etc. The Model S has a Wh/mi line graph interface available to be viewed on the touchscreen. Wh/mi is like, but not exactly like, gas mileage expressed in electricity terms.

Now we have a construct of knowing energy consumption when we drive an EV. Tesla Motors has gone to great lengths to calculate this very accurately and allows you to display it either in the LED panel immediately behind the steering wheel, or on the main touchscreen, or both. From this display we will see miles driven, kWh used and our Wh/ml. On a recent long trip I drove 162.2 miles and used 59.7 kWh.

Energy

Next we need to convert our kWh of electricity consumed into gallons of gas.

                                       Kw3

Using the above 162.2 miles and 59.7 kWh used we match electricity used to 1.77 gallons of gas if we were driving a gas car. Almost there. The final step is to calculate the Miles Per Gallon Equivalent referred to earlier in this post. This gets us to an apples to apples comparison of an EV to a gas car.

Kw4

The 162.2 miles I drove using 59.7 kWh is equivalent to 91.8 MPG. The window sticker of a new car has for many years been required to show MPG. Here’s the sticker from my Model S.

Model S Window Sticker

The MPGe is very close. I question the annual fuel cost of $650. In my opinion this is high, but then it depends on how many miles you drive and where you charge. When I charge at home, which is where it occurs probably 320+ times out of the year, my fuel cost is likely lower.

In my village I pay $0.04999 per kWh of electricity. This cost is not variable, meaning it doesn’t go up in peak times and down in the wee hours of the morning. My commute is 10 miles roundtrip. Yes, 10 miles. Actually less, but we’ll round up. I had  240 volt, 40 amp outlet installed with a NEMA 1450 plug in my garage. It takes roughly 20 minutes to replenish those 10 miles. According to Tesla that will use 3.3 kWh. Do the math. it costs me 16¢ in energy costs per day to drive my commute. If I were to drive that commute for 10 years I would spend $387 on electricity to recharge my Model S. No tailpipe emissions included.

I hope this helps you “on the fencers” out there to understand more about electric cars.

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