kilowatt hour per mile

kilowatt hour per mile

Eleven days ago I put a kilo-watt hour meter in line of the 50 amp charging outlet I use to charge my car which I have owned four months. I purchased the standard electric meter on-line from Hialeah meters in Miami. The meter and mount cost only $30.00 and was very simple to install. I have driven my car about 30 miles a day over the past ten days and charge it every night. I have found I get about 0.47 kwh/mile. That is equivalent to about 75 MPG which is much less than the 94 MPG EPA estimate. I believe most of the difference is due to the Vampire Loss while the car is idle. Does anyone know when Tesla is going to fix this? Until they do I don't believe the cars should be advertised as 94 MPG equivalent. Don't get me wrong I am happy with the car but I also believe in truth in advertising.

Has anyone else metered their charging outlet?

Thanks for any feedback.

Bighorn | 28. September 2013

How does the measured 470 Wh/m compare with your onboard computer data? Vampire loss is addressed in the 5.0 update that is being beta tested currently.

Bighorn | 28. September 2013

Also recognize that your efficiency is stilted by your relatively low daily mileage as a proportionally larger percentage of your energy use is going to "vampire loss".

Sudre_ | 28. September 2013

If you are driving the car and getting a 350 watt/mile then obviously your MPG equivalent is going to be less.

Try this.

take your charging 470watt/mile.
Subtract out your actual driving watt/mile or even your average.
Then add in 250 watt/mile which is close to what Tesla calls the Ideal watt/mile.
then see if the equivalent is closer to 94mpg.

for example: 470-350=120 +250=370
The 120 in this example is the actual charging losses.

If you are not driving an ICE efficiently you can't expect to get the same MPG as advertised so if you are not driving your S efficiently you won't get the same either.

You are right tho, it probably has a lot to do with vampire losses. That is corrected in 5.0 and they are testing it now with a few owners. Should be out soon.

RussellV | 28. September 2013

The onboard computer registers about 330 watt hour/mile. I understand the efficiency would go up if I drove more every day.

Thanks, Bighorn. What else is in the 5.0 update?

RussellV | 28. September 2013

Thanks Sudre.

Bighorn | 28. September 2013

I don't have 5.0 yet, but there are several threads about it. Heading up orientation on the GPS maps are one of the long-awaited features, but some cars' GPSs have been seriously hobbled by a bug in the works. Not ready for primetime. Reduction of vampire loss to 0-1 miles a day seems common at the minor expense of a slightly longer wake up period for both the app and ignition sequence.

Rubik | 28. September 2013

I find using one of the trip odometers helpful in understanding long-term usage trends, especially to understand a commute-week/month. Your 330 Wh/mi looks to be (33611 / 330) = 101.85MPGe

AmpedRealtor | 29. September 2013

I don't believe the EPA ratings factor in charging efficiency losses and vampire drain. I don't feel Tesla is being misleading by advertising a rating that they were given by EPA. Also, you can't expect a battery not to lose charge when idle. Even your AA rechargeable battery starts losing charge the moment you unplug the charger. There will always be losses that are not identified by the EPA ratings, the question is how to best minimize them.

Now let me do a practical and realistic MPGe calculation that uses the cost of gas instead of the amount of electrical energy contained in a gallon of gas (EPA's method). I drive approx. 20,000 miles annually. That will consume 6,415 kWh in energy (320 Wh/mile). Add another 8 miles per day for vampire loss and 15% for efficiency loss and we get 8,650 kWh per year. My time of use solar electric rate averages 6¢/kWh, so the cost of that power is $519 per year. The average cost of regular unleaded in Arizona is about $3.30/gal. The electrical cost of $519 to power a Model S for 20,000 miles will buy 157 gallons of gas at today's prices.

20,000 miles ÷ 157 gallons = 127 MPGe

James S | 29. September 2013

I know that the "best practise" is to charge every night, but I wonder of it is actually necessary when all you are doing is topping off the last 10% of charge?

Even with Vampire loss, you should be able to go for at least a week between full charges with your 30 miles/day usage.

At the end of that week, give it a full charge and see if your MPGe (including vampire losses) looks any better?

hikerockies | 30. September 2013

@RussellV - Since you quoted 94 MPGe, I assume you have a 60 kWh battery (S85 is rated at 88 MPGe in the city). EPA efficiency seems to be coming from 290 Wh/mile. Your 330 Wh/mile would imply you will see less than EPA rating. I have an S60. I drive about 15 miles/day over the weekdays and about 200 miles maybe every other weekend. Onboard computer on my car reads an everage of about 285 Wh/mile for me. I have been driving in the city with climate control turned off for the last few days just as an experiment (outside temp has been in 50-70 deg F range) and I have been seeing about 260 Wh/mile.

earlyretirement | 30. September 2013

@ Amped. Wow is your cost per kWh GREAT there! I'm looking at solar now. The lowest rates I'm getting in San Diego with a dedicated TOU meter in super off peak is 0.16 cents per kWh. I'd love to pay 6 cents! Kudos.

Cindy I II III | 01. Oktober 2013

AR, "regular unleaded" :-) last time I heard that was when I broke up with my ex 18 years ago. How long has it been before "leaded" was available? Your calculation method seems quite fair to me. Though many performance vehicles at MS price range use premium, which is about 20-40 cents more? But electro rate in NJ is bad, like 15 cents. Got to do something about it.

jat | 01. Oktober 2013

@earlyretirement - in GA, if you go with an EV rate plan you get $.019/kWh in the middle of the night. However, it doubles our peak rate to $.19/kWh, and I would lose far more than I gain by spending more on the rest of our electric usage. So, I just stick with our $.045/kWh (winter) and $.09/kWh (summer) rates.

Brian H | 01. Oktober 2013

Ya, a lot of people would love a discount rate as low as your high rate.

Roamer@AZ USA | 01. Oktober 2013

I totally stopped worrying or thinking about energy cost. With the Tesla, energy cost is so low a Starbucks visit costs more than than a 300 mile trip.

I bought my first electric car almost three years ago because my utility pays me 3.5 cents a kWh for my surplus solar power. I figured it was better to use it myself than sell it back. I net meter all year then every April they credit my annual surplus. The system is wonderful. Spring and fall I make lots of extra summer and winter I may use a little more than I make but never end up with a bill because I always have a positive balance on the meter.

So my Tesla energy cost is the lost opportunity to sell power back at the 3.5 cent wholesale price.

The Tesla gets 3 to 3.5 miles per kWh so I figure my cost at a penny a mile. Only way I can beat that is to use a super charger that does not yet exist in Arizona. For the dollar or two I would save at a super charger not sure it's worth killing 30 minutes just to get free electricity.

It's so cheap to enjoy the car I lost interest in figuring the math. Bigger things to worry about. Nice to have energy become so insignificant.

I plug my car in every time it's in the garage and always have 7/8 of a full tank. Gotta love having your car filled up every time you get in it and all from the sun that lands on your roof.

Unbelievable that you can be 100% energy self sufficient with your house and two battery cars.

I have to say....I am lovin it.

Roamer@AZ USA | 01. Oktober 2013

As far as metering I started out using the EV Project Blink chargers that I put in for my Leafs. I love that they track all kinds of charge data and store two months of data. If you really want to see what is being used they are great. You can put in your electric kWh cost and it also figures how much gasoline you would have used.

Hated to stop using them but the Tesla cord is so convenient, it opens the charge door, is smaller than the J1772 with the adapter and most important the Tesla equipment can really pump the amps into the car.

Someday Tesla will have better energy use info and calculation built into the car display like the EV Project Blinks. I was pretty let down at how basic the Tesla data reporting screens are after using the Blinks for several years. But at 1/2 the price the Leaf also has backup camera lines and a functional GPS system.

Gave up all the Leafs mature functions for the range and good looks of the Tesla.

Brian H | 02. Oktober 2013


wcalvin | 26. Februar 2014

Some cautions on AC trickle charging at 120V: about 2/3s is going to heat, up at the top end of the charging curve.

I was just figuring out what I owe my condo association for charging my 85. The actual watt-meter version works out to three times the charge delivered to the battery.

I am using the voltage, current, fractional SOC, and battery current from the charge logs via the VisibleTesla export, and I have checked the first two with a kill-a-watt meter in the 120v supply. So for a 17 hr session at 109V and 5A, the product gets us 9.3kWh used. From the boost in the battery, 0.04*80kWh=3.2kWh. (0.04 is the increase in the fractional charge using 1.0 as 80kWh.)

Mind you, I am limiting to 5A because I have a line resistance of 2 ohms and 7A gets me too close to the 105 cutoff for the operating range. So don't expect this 3X draw to represent other places down the charging curve.

I assume this is all energy being spilled as heat by the 10kW AC charger, to get 5A down to less than 1A.

Anyone done an analysis of this? That's a lot of waste.

DonS | 26. Februar 2014

Agreed, that is a lot of waste. Note that the car does not sleep when charging, so part of the power goes into the vampire load. Also, 120V charging is less efficient than 240V charging. So, 120V (or 109V)at 5A is the least efficient possible way to charge.

Brian H | 26. Februar 2014

Charge the condo for heating the garage.

ye | 02. März 2014

wcalvin, I would have guessed that the car's charger is more efficient than that. I wonder if maybe the car is intentionally using some electricity to heat the battery because it's not good to charge a cold battery. Is it very cold where you are?

LMB | 02. März 2014

(LMB spouse)

@wcalvin - Also, the car does not convert the 5 amps to 1 amp by "burning" off the other 4 amps. It has an AC to DC converter so roughly: 5 amps at 120 volts AC is converted to 1 amp at 400 volts DC. The converter is designed to be efficient at medium to high charge rates, but you are using 6% of the charger capacity and I would not expect much efficiency at that level. And, as others have said, your charging current is not much higher than the car's non-sleep power draw.

EVTripPlanner | 16. März 2014

There are multiple factors that impact real-world meter-to-wheels energy usage. You can see my write up and calculations at

Bottom line: depending on how much you drive and whether you set the energy saver sleep mode, you can use more than twice the electricity that is reported by the in-car metering.

huzz1970 | 16. März 2014

Power converters (my company makes the components) are about 85% efficient (give of take details of the circuit topology). This efficiency is PEAKING at about 50% of the rated load. It is much lower at light loads and also the efficiency decreases at high loads. There will be some improvements in the next few years as people move out of silicon components to higher performance electronics, but not in the next 1-2 years. However, these circuits will still peak around 50-60% load, probably with a bit less reduction at light and heavy loads. So, if your meter is showing ~15% "loss" this is expected from the wall plug to the batteries.

Czech | 17. März 2014

I remember seeing somewhere that a 110v plug is about 86% efficient and a 240v is about 93 or 96% efficient give or take a few percent for memory.