An Energy Primer And My Tesla Energy Usage So Far

(Not actually from my car)

I remember back when I first got my Prius, I was totally fascinated with gas mileage and how to maximize it. There is a whole community dedicated to hypermiling - trying to get the maximum miles per gallon from their cars. After owning a Prius for a while, my interest in gas mileage gradually tapered off and I just accepted that I would get between 48 and 53 miles per gallon. Tesla owners are no different only instead of miles per gallon, they geek out over watt hours per mile.

There are probably not many subjects drawing the attention of Tesla owners, especially new Tesla owners, more than energy usage. A Tesla keeps all sorts of statistics about its energy usage and a lot of that information is made available to programmers via Tesla’s API. This allows for programmers, be it third party developers or just amateur hobbyists, to look at raw data from the car and go full nerdcore on all sorts of technical information.

It’s The Internet. Of Course, People Are Arguing!

In reading Tesla owner’s posts about the subject, it’s clear this can be quite the rabbit hole and I have no intention of falling in. There is a wide range of disagreement on how much power a Tesla actually uses and ways to compare how much power actually gets into the battery versus what is actually drawn from the power company (which isn’t the same due to heat and resistance loses, efficiency losses, battery degradation, etc.), what the best current level to charge at is, as so on. And don’t even get me started on the semantic argument over wall charger versus wall connector! I’ve got a degree in electrical engineering and even I don’t want to get involved with these discussions.

I Don’t Care If I’m Off By A Tiny Bit

Instead, I’m just going to go with what the car reports. This may be slightly different from what is actually used, but trying to figure out anything in more detail involves making a whole lot of assumptions of information we just don’t have.

Look at this like trying to calculate a gasoline car’s miles per gallon. You fill the tank, reset your tripmeter, drive around, fill the tank again, and note your mileage. Your miles per gallon is the number of miles your tripmeter reports divided by the number of gallons of gas you put in. Simple, right?

The problem is that’s not going to be super accurate. Your tripmeter is calibrated for a certain diameter wheel and tire. Is your wheel the same size as what the manufacturer calibrated with or did you get custom wheels? What about the tires? Did you know different tires have different rolling resistances and that rolling resistance may account for 5% to 15% of fuel consumption? Tire pressure affects mileage, so if your tires change pressure (and they do with temperature, as well with possible leaks), you can’t get an accurate number. The density of gasoline varies with temperature, so if you initially filled your tank on a hot day then filled it again on a cold day, you might be putting more gas into your tank than you actually burned.

In other words, when you figure your car’s miles per gallon, you ignore a whole bunch of variables that may affect your final number and you just use the data the car reports to you (via the tripmeter and the gas pump). I’m going to do the same with my electric car and avoid falling down that rabbit hole I mentioned earlier.

I’ve linked my Tesla account to the EVTripping website. This allows the website to get an authentication token that allows it to connect to my Tesla and download data each night. I’ve had an account with them since I bought my car, so they’ve got my entire usage history from Day 1.

The MPG Equivalent For Electric Cars

The important number EV owners track is watt hours per mile (Wh/mi) – how much energy it takes to move the car per mile. The lower the number, the better. Just like it assigns gas cars a miles per gallon figure, the EPA rates electric cars with an energy usage value. They rate my car at 35 kWh/100 miles . That is another way of saying 350 Wh/mi because 1 kWh = 1,000 Wh.

Many of the apps that track Tesla energy usage calculate an efficiency number. This number is how your car compares to the EPA rated performance. In gas car terms, this is like comparing your actual gas mileage with the EPA stated gas mileage for your vehicle. If the EPA says your car gets 25 miles per gallon and your actual figure was 20 miles per gallon, you could say your efficiency was 80%, or 20 divided by 25.

A similar calculation is done by most Tesla-related apps. Your efficiency is your actual energy usage divided by the EPA rated energy usage. Numbers greater than 100% indicate you are using less energy than the EPA calculated you would.

It’s incredibly rare for a gas car to get better gas mileage than the EPA calculated value. After all, their tests are done under ideal conditions and your driving isn't. Further, with gas cars, fuel flows in one direction – from your tank through the engine. By that, I mean if you are going downhill, you may use less gas, but you don’t actually get more gas in your tank as you travel down the hill.

Wait, I Get Power Back?

Thanks to regenerative braking, electric vehicles do reverse the energy flow and refill your “tank” (aka battery pack)! As gravity pulls your car down the hill, your motor switches from consuming energy to generating energy and feeds it back into your batteries. In effect, this lets gravity “refill your tank.”

Now, there is no free lunch. Going down a hill will never allow you to recapture *all* of the energy used going up the hill. You lose energy due to friction, heat, wind resistance, and a bunch of other factors. However, going down the hill can regain you around 90% of the energy used to go up the hill in the first place. (Again, this is a number many people argue about, but it’s what most trip planning sites use when calculating energy usage over changing elevations and it seems to match well with people’s experiences, so I’m going with it.)

Being able to generate their own electricity is what helps electric cars achieve greater than 100% efficiency. For example, look at this data from my drive to and from work:

The trips are listed in reverse chronological order, so the bottom trip is my drive to work and the top is my drive home. You’ll notice my drive to work has an efficiency of over 100%. Part of this is due to the change in elevation. My home sits at 375 feet above sea level and my work is 138 feet above sea level. My morning commute is basically a 237 foot downhill drive. (For those sharp-eyed readers looking at the distance figures, I take different routes to and from work to avoid traffic, which is why the mileage is slightly different for the trips.)

I also use Autopilot in both directions, which helps reduce energy consumption by reducing the small fluctuations in speed that happen naturally with human drivers because they can't keep their foot pressure perfectly constant. (Cruise control in gas cars provides a similar boost to MPG.)

My Numbers

After two weeks of ownership, here are my lifetime energy usage numbers. Note that this chart lists efficiency as Wh/mile rather than a percent comparison to the EPA rated number (which again, is 350 Wh/mile).

My average efficiency is better than the EPA stated 350 Wh/mi. Granted, this is only after two weeks of use and, as you can see, there is no battery degradation. Over time, batteries do start to lose some of their energy storage capability. Real world testing by Tesla users has shown that I can expect about a 5% loss after 50,000 miles, but the loss really tapers off after that. Tesla battery packs should retain 90% of their capacity after 160,000 miles and 80% after 500,000 miles!

For comparison, with my Prius, I did not notice any battery degradation after 8 years and 145,000 miles. Granted, the Prius provided absolutely no way to measure battery usage other than a 6 segment image on the dash, so it’s not likely I would have noticed a 5% to 10% loss in capacity.

In short, I’m not worried about battery life.

Next week, I’ll talk about energy costs and compare it to what I would have spent on gasoline.

If you have questions or comments, be sure to leave a note below!