Wh/mi and why should I care?

[theslimshadyist]

Ok guys, this is a serious question and because I'm from TN and a little slow, I wanted to obtain a simplified answer and not some Einstein mathematical equation that I would need to solve.

I see a lot of chatter around Wh/mi and all sorts of numbers and formulas and yes, I do understand that there are 1000 watts in a kilowatt so allow me to proceed. Since the remaining range is displayed directly in front of me, why would I really need to be concerned about Wh/mi calculations and conversions?

 

[DarkMatter]

You don't need to be. If you don't care, don't. It's just the equivalent to gas MPG. People care a bit more on electric because the 'tank' stores less energy and takes longer to refill it. But if you weren't the sort of person who tracked your MPG on a previous car you don't need to care on a Tesla.

 

[theslimshadyist]

You don't need to be. If you don't care, don't. It's just the equivalent to gas MPG. People care a bit more on electric because the 'tank' stores less energy and takes longer to refill it. But if you weren't the sort of person who tracked your MPG on a previous car you don't need to care on a Tesla.

Ok, that makes sense considering that my previous vehicle was a twin turbo V8 and typically got 18 in city and 25 on the interstate, I didn't really have a reason to be concerned with fuel efficiency because it was less than stellar and I didn't purchase for that use case.

However, if I wanted to calculate an equivalent to gas MPG, what simple formula would I use to accomplish this?

Also, if my math is correct and it usually is not, but to use a simplified example if my utility company charges $.10 per kWh would that basically cost me $10 to go from 0-100?

 

[johnnyS]

In day to day driving you should not care. On road trips it may matter when you are trying to make the next charging station. Since getting the Tesla we added solar panels. Essentially I invested in my future electricity. It really does not matter how efficient I drive since sunshine does not cost me anything. Everyday I have a full tank so I have no worries about watching efficiency.

 

[SomeJoe7777]

The biggest thing you need to keep in mind is that the displayed range is the rated range. In other words, that's the range you would get if you drove the car in efficient conditions, obeying the speed limit, and drove it gingerly.

But of course in the real world, many things take you away from those ideal conditions. Driving faster than the speed limit, driving in cold weather, spirited driving, weather and wind, etc. will all take away your efficiency.

The rated range displayed at the battery meter is what you would get if you drive such that you are using about 280 Wh/mile. But if you're driving on the highway above the speed limit into a headwind, you might be using 350 Wh/mile. That's about 25% more than 280. So you will get 25% less range than what's shown at the battery meter.

This is quite similar to gasoline car MPG ratings. Those ratings are for idealized conditions. A car might be rated to get 30 MPG on the highway, but if you do 75 MPH into a headwind, you're only going to get 26 MPG. Same thing here.

 

[DarkMatter]

Ok, that makes sense considering that my previous vehicle was a twin turbo V8 and typically got 18 in city and 25 on the interstate, I didn't really have a reason to be concerned with fuel efficiency because it was less than stellar and I didn't purchase for that use case.

However, if I wanted to calculate an equivalent to gas MPG, what simple formula would I use to accomplish this?

Also, if my math is correct and it usually is not, but to use a simplified example if my utility company charges $.10 per kWh would that basically cost me $10 to go from 0-100?

OK, there's MPGe, but it's kind of useless. That's why everyone uses Wh/mi instead, since it's actually useful. 100kWh battery, getting 300 Wh/mile? You're like to get around 300 miles. (100 divided by .3 is 333) So it's the useful equivalent to track. To calculate it just divide the miles in to the kW used and then multiple by 1000. Try it on the above figure to verify that you know the math.

If you want to convert to MPGe a site like this will do the math for you: Fuel economy conversions / calculator - EcoModder.com
It shows that your 18MPG would be the same as getting 1878 Wh/mi. 300 Wh/mi would be 112 MPGe. You can tell how that number is really pretty useless.

Now to determine cost/mile just take the number of kW used by a trip and multiply it by the rate you're paying for electricity. So that same 300 miles which took you around 100 kW would cost about $10 worth of electricity at 10 cents/kW. You can see how this is cheap enough compared to the cost of the car that most people just don't track it.

 

[sorka]

Even on long trips, you can just use the energy trip graph which will project your energy usage on a curve to the the next end point of your trip and as you drive, it will adjust the actual curve in relation to the original projected curve so you can see if you're doing better or worse than the original projection. If the end point project curve dips below say 10% .... or 5% if you're feeling brave, then you know you need to slow down or turn off the AC/heat or both in order to make your next destination. It's only the geeks like us that actually look at wh / mile. It's not really needed.

 

[jaguar36]

The most useful thing about the wh/mi display is for seeing how much below the "Rated" range you'll get. If you're up in the low 400wh/mi you'll only get about 245 miles of real range on a P100D. If you somehow manage to keep it around 300wh/mi, you'll actually get around 325miles of real range. Keep in mind the P100D has somewhat less than 100Kwh of usable battery, probably around 92Kwh although I don't think anyone has really determined it accurately yet.

The ecomodder calculator converts the wh/mi to MPG based on the actual energy in a gallon of gas, which is a pretty worthless calculation. Whats far more important is an equivalent MPG based on how much each costs. The formula for that is:

(Price of gas)/((Electricity cost per kwh)/(1000/((Tesla Wh/mi)/(Charging Efficiency))

So for instance, if gas is $3.00 a gallon, electricity is $0.19/kwh, charging efficiency is 90% (Tends to be somewhere between 85% and 95%) and you're getting 350wh/mi, that works out to:

3.00/(0.19/(1000/(350/0.90)))= 40MPGe

 

[SageBrush]

Since the remaining range is displayed directly in front of me, why would I really need to be concerned about Wh/mi calculations and conversions?

It depends on what you want to know.

• E.g., knowing you make a trip without running out of fuel before you start out.
• Or knowing that your car is not running at it's usual performance level.
• Or realizing that consumption is a lot higher than the car realizes, e.g from wind or bad roads or climbs and the estimated range is way off.

I suppose the important thing to realize is that calculated range is based on a set amount of past performance. That may be way off compared to what is in front of you.

 

[theslimshadyist]

The biggest thing you need to keep in mind is that the displayed range is the rated range. In other words, that's the range you would get if you drove the car in efficient conditions, obeying the speed limit, and drove it gingerly.

Surely the displayed range will fluctuate while driving the vehicle and adjust based on current driving conditions on a long trip? For example, when I leave the house the range shows me 310mi, I get on the interstate and drive 80, then the range meter shows I have 200mi of range, then I slow down to 65 for let's say drive for 20mi, wouldn't the range meter readjust and go back up?

 

[DarkMatter]

Whats far more important is an equivalent MPG based on how much each costs. The formula for that is:

(Price of gas)/((Electricity cost per kwh)/(1000/((Tesla Wh/mi)/(Charging Efficiency))

So for instance, if gas is $3.00 a gallon, electricity is $0.19/kwh, charging efficiency is 90% (Tends to be somewhere between 85% and 95%) and you're getting 350wh/mi, that works out to:

3.00/(0.19/(1000/(350/0.90)))= 40MPGe

That's more like miles/dollar just shown in a strange way. I would not call it more important unless cost is the thing you're trying to measure rather than efficiency, in which case you probably just want straight cents per mile for each technology as that will be an easier number to use.

 

[theslimshadyist]

My main use case is making sure that I have information available to me when taking a long trip and knowing that I will get to my next way point or destination without running out of juice.

 

[kort677]

My main use case is making sure that I have information available to me when taking a long trip and knowing that I will get to my next way point or destination without running out of juice.

that is already covered by the in car trip planner app, in conjunction with the nav system the display will show your wh/mile usage and will show if you are withing range capabilities or exceeding the SOC necessary to reach your destination.

 

[SomeJoe7777]

Surely the displayed range will fluctuate while driving the vehicle and adjust based on current driving conditions on a long trip? For example, when I leave the house the range shows me 310mi, I get on the interstate and drive 80, then the range meter shows I have 200mi of range, then I slow down to 65 for let's say drive for 20mi, wouldn't the range meter readjust and go back up?

No, the rated range meter on the instrument cluster does not do that. It always shows rated range based on efficient conditions.

If you pull up the energy graph on the center display, on the right is a "predicted range" which behaves as you describe. It varies depending on the last 5, 15, or 30 miles of actual driving conditions (based on which setting you have selected for the graph).

I find it useful on long trips to compare the miles remaining on the trip vs. the predicted range. Watching these two numbers over time can tell you if you're making up ground or losing ground with regards to your charge state at the end of the trip, and ensure you will still arrive with a suitable buffer.

For example, lets say I plot a trip, and I'm predicted to end up at 15% battery at the end according to the trip planner. Midway through the trip I have 70 miles to go vs. 100 miles predicted range. That indicates that I should have a 30 mile buffer in the battery when I reach my destination. But lets say 10 miles later, I have 60 miles to go, and now 85 miles predicted range. My buffer at the destination has now fallen to 25 miles, so I'm losing ground. Slow down, turn off heat, etc. until the buffer stabilizes or begins to increase again.

 

[Model S M.D.]

This number is useless on a P100DL, trust me and enjoy!

(its very hard not to step on the accelerator at every opportunity)

 

[democappy]

As others have noted, 99% of the time you shouldn't care at all about Wh/mi. That said, the inner statistician in me seems to care deeply about it while I am driving even though it has never actually mattered in any real tangible way to me. It is almost like a scoreboard showing how good/bad of an efficient driver I am being. Even though I don't care about how efficiently I am driving I just can't help but look at the score and think about why it is high/low.

You can set the car to show actual miles versus rated. The navigation and the general how I have been driving the last 5/15/30 miles screen can show a version of truer mileage left.

 

[steveho]

One thing that always bothered me is that MPG is really miles per gallon of gas. Whereas Wh/Mile is watt-hours per mile. We should actually change MPG into GPM to rate gasoline cars. I know it's just an inverse function, but so many people think that 50MPG is really really better than 45MPG, when in reality it is only 10% difference. In gallons per mile, or gallons per 1000 miles, so 22 MPG = 45 gallons per 1000 miles.

We don't think of EVs in terms of Miles per Wh (or KWh)! 3 miles per 1kwh.

 

[RogerHScott]

But if you're driving on the highway above the speed limit into a headwind, you might be using 350 Wh/mile. That's about 25% more than 280. So you will get 25% less range than what's shown at the battery meter.

This is a common mathematical fallacy. It is obvious that it is wrong if you change "25" to "100": doubling the rate of consumption does
not result in no (i.e., 100% less) range. A 25% increase in consumption results in a 20% reduction in range. A 100% increase in
consumption results in a 50% reduction in range. Sparing y'all the real math, a 1/N (N=1,2,3,4,...) increase in rate of consumption
corresponds to a 1/(N+1) reduction in range.

 

[RogerHScott]

We don't think of EVs in terms of Miles per Wh (or KWh)! 3 miles per 1kwh.

Lightyears per gigawatt-hour all the way!

 

[Brass Guy]

One thing that always bothered me is that MPG is really miles per gallon of gas. Whereas Wh/Mile is watt-hours per mile. We should actually change MPG into GPM to rate gasoline cars. I know it's just an inverse function, but so many people think that 50MPG is really really better than 45MPG, when in reality it is only 10% difference. In gallons per mile, or gallons per 1000 miles, so 22 MPG = 45 gallons per 1000 miles.

We don't think of EVs in terms of Miles per Wh (or KWh)! 3 miles per 1kwh.

I think in Europe the formula they use is liters/100km. It would make it easier to make the switch to Wh/km. Those Europeans, always trying to make things easier. Sheesh.