Help understanding Wh/mi (EPA vs. displayed)

[krazineurons]

I've had my Model S since August, and am still having a bit of trouble getting a sense of how much energy I'm using. With my old Volt, it was easy: at the end of a trip, you could see miles used, estimated miles left. If I drove 25 miles and had an estimated 10 miles left, that's about 35 miles compared to the EPA estimate of 38: I did slightly better than the EPA average.

With the Tesla, things get trickier. It doesn't show estimated miles left (just EPA miles left or ideal miles left, which are really just different ways of displaying % of battery left), and the battery is so much bigger that for shorter drives the math isn't as accurate.

Issue 1: The EPA for an 85kW Model S shows 370-380Wh/mi (.37-.38 kWh/100mi), and a 265 mile range. Those numbers do not compute. 85kW divided by 265 miles is 320Wh/mi. The EPA shows a range 15% lower than what it should be -- is that because it either [1] assumes people charge to 85%, [2] 15% of the battery is "hidden" when charged to 100% (not used. or [3] something else?

Issue 2: Here's what I saw today: I drove 14.9 miles, used 5.1kW, at 341kWh/mi. I used 18 rated miles (going from 199 to 181 rated miles, which could really be ~17.1 to 18.9 rated miles). So according to the display, I used about 20% more than the EPA estimate (I used 18 rated miles worth of electricity, but only actually drove 14.9 miles: 18/14.9=1.2). But that suggests the EPA is 284Wh/mi.

So I'm seeing three different EPA values (~284Wh/mi, 320Wh/mi, 370-380Wh/mi), and am lost. Any ideas?

Can you tell me where you found the 370-380 values marked as official rated values? It seems the official rated should be around 320 and that makes sense for 265 rated miles.

As others have said, it's best to stop computing off the rated range, it's a scam. Here are three things to watchout for and see if it makes sense

1) Set the battery display to be % and not rated or ideal.
2) Use trip meter and look at since last charge and custom trip meter numbers. They reflect based on your usage and nothing else. Try to charge the battery to 90%, reset the meters, go on a trip, calculate the remaining % vs the kwh consumed, it will give you an idea of how. much 10% battery is holding. In winters my X averages 440Wh/mile vs a rated of 330Wh/mile so if I estimate my usage based on that it always comes up correctly. For a 90D I have around 8KWh per 10% battery.

 

[Saghost]

Can you tell me where you found the 370-380 values marked as official rated values? It seems the official rated should be around 320 and that makes sense for 265 rated miles.

As others have said, it's best to stop computing off the rated range, it's a scam. Here are three things to watchout for and see if it makes sense

1) Set the battery display to be % and not rated or ideal.
2) Use trip meter and look at since last charge and custom trip meter numbers. They reflect based on your usage and nothing else. Try to charge the battery to 90%, reset the meters, go on a trip, calculate the remaining % vs the kwh consumed, it will give you an idea of how. much 10% battery is holding. In winters my X averages 440Wh/mile vs a rated of 330Wh/mile so if I estimate my usage based on that it always comes up correctly. For a 90D I have around 8KWh per 10% battery.

2012 Tesla Model S

In the EPA fuel economy block in the middle of the page, the fine print under the MPGe rating says 38 kWh/100 miles - which is 380 Wh/mile.

89 MPGe = 89 miles per 33.7 kWh -> 33.7 kWh/89 miles = 378 Wh/mile.

 

[dknisely]

Forget the magic range estimates. I want to know what the scale for instantaneous consumption/regen is (labeled "W," but clearly not Watts)... I've never found any explanation. It goes from 0 to something like 200, with 50 being significant acceleration and 100 being very high consumption. The units are apparently conceptual; they don't make any sense interpreted as Amps or Watts.

 

[Saghost]

Forget the magic range estimates. I want to know what the scale for instantaneous consumption/regen is (labeled "W," but clearly not Watts)... I've never found any explanation. It goes from 0 to something like 200, with 50 being significant acceleration and 100 being very high consumption. The units are apparently conceptual; they don't make any sense interpreted as Amps or Watts.

I'm pretty sure it's kW of power.

 

[rscott0]

As others have said, it's best to stop computing off the rated range, it's a scam.

It's not a scam, it's a benchmark. It isn't perfect, but it's the only one we have.

Specifically, people often use the rated range of the Tesla. In my case, it's 265 miles. So if I start at 80% and I've driven 50 miles, and it says I have 160 miles rated range, how will I know if I likely have that 160 miles left, or more, or less?

The key here is the 295Wh/mi that Tesla apparently uses internally. If it says I have 160 miles left, I'll very likely get 160 miles *if* I can stay at or under 295Wh/mi (as the Tesla reports).

 

[csanders90D]

2012 Tesla Model S

In the EPA fuel economy block in the middle of the page, the fine print under the MPGe rating says 38 kWh/100 miles - which is 380 Wh/mile.

89 MPGe = 89 miles per 33.7 kWh -> 33.7 kWh/89 miles = 378 Wh/mile.

That 380 number is measuring the amount of electricity coming out of your outlet, the amount that the power company will bill you for. Some of that is lost in the AC->DC conversion, some is lost in battery charging, leaving around 285 usable in the battery. The "EPA Rated" usage and range numbers displayed in the car are based on the 285 Wh/mile number. (Your car's rating might not be exactly 285, it varies by model and options. P models are higher, 21' wheels are higher, etc.)

 

[Saghost]

That 380 number is measuring the amount of electricity coming out of your outlet, the amount that the power company will bill you for. Some of that is lost in the AC->DC conversion, some is lost in battery charging, leaving around 285 usable in the battery. The "EPA Rated" usage and range numbers displayed in the car are based on the 285 Wh/mile number. (Your car's rating might not be exactly 285, it varies by model and options. P models are higher, 21' wheels are higher, etc.)

I thought the classic 85 kWh RWD rated number was a little higher than 285 Wh/mile, but you're basically right. I was just explaining where the 380 number came from to the poster I quoted.

As I said upthread in a previous post, the EPA numbers are different from the in car numbers because of the charging losses, just as you noted.

 

[Snerruc]

I don't understand you guys complaining about the rated miles. It is calculated from the EPA test just like the mpg for ices. In fact it is conservative compared to the ice one. My last trip in my 75 was 250 miles, and I came home with 32 miles in the battery. Trip included climb/ descents of 4000' and 6500'. Rated is only 249 mi. Color me happy. I have beaten EPA in ices rarely and never by this much.

 

[somnambule]

As others have said, it's best to stop computing off the rated range, it's a scam. Here are three things to watchout for and see if it makes sense

1) Set the battery display to be % and not rated or ideal.
2) Use trip meter and look at since last charge and custom trip meter numbers. They reflect based on your usage and nothing else. Try to charge the battery to 90%, reset the meters, go on a trip, calculate the remaining % vs the kwh consumed, it will give you an idea of how. much 10% battery is holding. In winters my X averages 440Wh/mile vs a rated of 330Wh/mile so if I estimate my usage based on that it always comes up correctly. For a 90D I have around 8KWh per 10% battery.

Do people really worry so much about this? Personally, for everyday driving, I just make sure there's plenty of range and not worry about it. After all, the S is a fun car to drive and I want to have fun with it and not obsess about range. On longer trips, I rely on the projected mileage based on the last X miles and the projected remaining charge % at destination from the nav. I like the KISS principle .

 

[rscott0]

Do people really worry so much about this? Personally, for everyday driving, I just make sure there's plenty of range and not worry about it. After all, the S is a fun car to drive and I want to have fun with it and not obsess about range. On longer trips, I rely on the projected mileage based on the last X miles and the projected remaining charge % at destination from the nav. I like the KISS principle .

For me, there are two reasons to worry about the EPA estimate:

[1] Today, I drove about 28 miles, and the car reported 310Wh/mi. Is that good or bad? Before this thread, I had no clue. Now that I know that 295Wh/mi is (reportedly) hard-coded as the EPA estimate, I know I did very close to the EPA estimate (which I thought was pretty good given the weather, speeds, etc.).

[2] About a month after getting the Model S back in August, I took a ~400 mile trip. It isn't easy to avoid range anxiety with a trip like that (e.g. what if a charging station isn't available?). Despite using evtripplanner.com, I had *no* clue on the trip how well we were doing. Sure, it says 105 miles left (or 40%), but it's hard to get a grasp without having *something* to compare it to. If I took that same trip today, I would know that if it says I'm getting 280Wh/mi, a 100% charge will get me over 265 miles. It's reassurance.

 

[David29]

Here's what I am thinking. All I really care about is how my driving compares to the EPA. I cannot tell what my expected range is with the Tesla, like I could with the Volt. But I can see Wh/mile.

So I will ignore the EPA kWh/100mi (Wh/mi). The EPA 265 mile range matches (I believe) what the car will show if the battery were at 100% (which is technically a bit less than 100%, but I'm willing to ignore that behind-the-scenes magic).

The Tesla uses 295Wh/mi for its calculation, and it magically matches the EPA 265 mile range. I won't think about why the 295Wh/mi isn't what I would have thought, but instead take that as if it were the official EPA number.

That gives me what I need: if I get that 295Wh/mi going from 100% to 0%, I've driven 265 miles. So if I drive around town and it shows 325Wh/mi, I've done slightly worse than the EPA estimate; if it shows 275Wh/mi, I've done slightly better.

So that 295Wh/mi number is the one that drivers need to think about, if they care about EPA numbers, and how their driving compares.

Bingo, I think you got it! That is also how I think about for my 70D. The EPA range of 240 computes to 291.7 (call it 292) wh/mi, so that is the figure I use as a basis for comparison. I also ignore the stuff about what portion of the battery is not usable, etc. I can occasionally achieve the EPA rated energy use (and sometimes even less) in nice weather when there is little demand for heat or AC and when I am on a trip of reasonable length. Cold, extreme heat and short trips all take much more, of course. Most of the time in daily use, I simply do not worry about how I am doing compared to the 292 milestone -- I just make sure I have plenty on the "tank" for my daily jaunts.

I do agree with the advice to set the battery gauge to percentage rather than miles. And I do find it useful to use the navigation system in conjunction with the energy app to see projected energy remaining on trips, even local trips around the Boston area. For longer trips, energy use and efficiency become more important, and having a working feel for the numbers is useful in trip planning. (Unfortunately for me, I have been inconsistent about collecting data on my energy use. I was more careful in the first year I had the car but have not tracked my energy use very consistently this year. In particular, I failed to track the energy we used for our first really long trip last fall, to Washington DC and back. I would really like to have seen how that worked out!)

 

[somnambule]

[2] About a month after getting the Model S back in August, I took a ~400 mile trip. It isn't easy to avoid range anxiety with a trip like that (e.g. what if a charging station isn't available?). Despite using evtripplanner.com, I had *no* clue on the trip how well we were doing. Sure, it says 105 miles left (or 40%), but it's hard to get a grasp without having *something* to compare it to. If I took that same trip today, I would know that if it says I'm getting 280Wh/mi, a 100% charge will get me over 265 miles. It's reassurance.

For me, the remaining charge % at destination is the key metric on longer trips. As long as that % is staying stable or going up, I know I'm doing fine. If it starts to drop and there isn't much margin, I try to improve the efficiency of my driving.

Of course, most of these projections break down if you need to go uphill or encounter significant changes to driving conditions. Being in California, I also don't know how reliable the remaining % projection is in cold weather. But, so far, it's worked well for me without having to do a lot of complex math or remember Wh/m thresholds etc.

 

[David29]

For me, there are two reasons to worry about the EPA estimate:

[1] Today, I drove about 28 miles, and the car reported 310Wh/mi. Is that good or bad? Before this thread, I had no clue. Now that I know that 295Wh/mi is (reportedly) hard-coded as the EPA estimate, I know I did very close to the EPA estimate (which I thought was pretty good given the weather, speeds, etc.).

[2] About a month after getting the Model S back in August, I took a ~400 mile trip. It isn't easy to avoid range anxiety with a trip like that (e.g. what if a charging station isn't available?). Despite using evtripplanner.com, I had *no* clue on the trip how well we were doing. Sure, it says 105 miles left (or 40%), but it's hard to get a grasp without having *something* to compare it to. If I took that same trip today, I would know that if it says I'm getting 280Wh/mi, a 100% charge will get me over 265 miles. It's reassurance.

Your 310 WH/mi from today is excellent in winter. Of course, yesterday and today have been warm. (Tomorrow the consumption could be a lot higher if the temps drop as predicted.)

My first long trip after I got my 70D in 2015 was from Dedham to Camden, ME, in early December. So like your long trip, it was a bit more than 400 miles round trip. About half of the trip was on I-95 and similar highways, and the other half on smaller roads such as Rte 1 and other 2-lane roads in Maine, at lower speeds. Temps were in the 30-55 range over the 2 days of the trip. I was pleasantly surprised to average 302 wh/mile, only 4& above the EPA rated average. So despite some energy used for heat, that shows the EPA rating can be achieved.

 

[rscott0]

Issue 1: The EPA for an 85kW Model S shows 370-380Wh/mi (.37-.38 kWh/100mi), and a 265 mile range. Those numbers do not compute. 85kW divided by 265 miles is 320Wh/mi. The EPA shows a range 15% lower than what it should be -- is that because it either [1] assumes people charge to 85%, [2] 15% of the battery is "hidden" when charged to 100% (not used. or [3] something else?

I got a response from the EPA yesterday.

The answer is #3: the EPA runs a test from a fully charged battery to depletion, and then uses AC kWh's to determine the Wh/mi. So presumably if you could magically stuff electrons into the battery without any loss, 320Wh/mi would be accurate.

This does get a bit dicey, as it factors in the loss from the outlet to the battery, which depends on a number of factors (amps, temperature, cord length, etc.).

 

[TIppy]

I have a P90DL, and according to canbus data I have 84.5 kwh total and 81.1 kwh usable. If you look on the power graph in the car, there is a dashed line that represents your epa average wh/mile (not including charging losses). The line for my car is at about 313 wh / mile. Tesla states that the epa range is 270 miles. 84500 / 270 = 313 wh / mile, and 81100 / 270 = 300 wh / mile. So you see the epa lets them use the total capacity of the battery even though 4 percent is held out for anti-bricking. That means that even if you consume the epa average wh / mile you will only get 0.96 * 270 = 259 miles of range. As a check, 81100 / 313 = 259 miles.

 

[DB 2]

I have a P90DL, and according to canbus data I have 84.5 kwh total and 81.1 kwh usable.

What are you using to get the canbus data?

I use TM-Spy and I am not aware of any CAN messages that include a gross pack capacity (that includes a truly unusable portion). The TM-Spy main screen shows a pack capacity when full, but all of that is usable, albeit several kWhs are after the UI reports zero miles (3.8 kWh in my car to be exact).

 

[TIppy]

Can ID 0x382 indicates a 4 percent anti-bricking energy buffer. Here is a TM-Spy screen grab of my car after I had zero miles left on the display. Note the 3.9 percent SOC in the lower left. Also at the top it has the pack capacity as 84.5 kwh and remaining capacity as 3.4 kwh. 3.4 / 84.5 is 4 percent. So usable is 84.5 - 3.4 = 81.1 kwh. The 84.5 kw is the nominal full pack energy.
There is some argument about whether you have extra miles below zero. wk057 claims this is an anti-bricking buffer and does not supply extra below zero range. At any rate, if the display says zero miles you would have only used 96 percent of the full pack capacity and would have only traveled 96 percent of the epa 270 mile range at the epa average wh / mile. That's 259 miles, not 270.

 

[DB 2]

Can ID 0x382 indicates a 4 percent anti-bricking energy buffer. Here is a TM-Spy screen grab of my car after I had zero miles left on the display. Note the 3.9 percent SOC in the lower left. Also at the top it has the pack capacity as 84.5 kwh and remaining capacity as 3.4 kwh. 3.4 / 84.5 is 4 percent. So usable is 84.5 - 3.4 = 81.1 kwh. The 84.5 kw is the nominal full pack energy.
There is some argument about whether you have extra miles below zero. wk057 claims this is an anti-bricking buffer and does not supply extra below zero range. At any rate, if the display says zero miles you would have only used 96 percent of the full pack capacity and would have only traveled 96 percent of the epa 270 mile range at the epa average wh / mile. That's 259 miles, not 270.

CAN ID 0x382 reports energyBuffer in kWh, not percent (see page 8 here). I suspect you had driven 1 to 1.5 miles after the zero level before you took that screenshot. Here is a easy way to see what your BMS SoC % would be at the zero level without actually driving that low again:

Set the charge level to 50% (the lowest setting) and charge.

When finished, use TM-Spy to get BMS SoC %.

Subtract 50 from BMS SoC % and then multiply by 2.

I think your BMS SoC % at charge level 50% will be at least 52.3%, meaning at least 4.6% at the zero level.

Regarding the extra miles below zero debate, I must disagree with @wk057 since I have tested it (more than once). See my post here for details, including photos/screenshots.

 

[wk057]

Regarding the extra miles below zero debate, I must disagree with @wk057 since I have tested it (more than once). See my post here for details, including photos/screenshots.

People really need to stop spreading the misinformation that there are always going to be "miles below zero." It is a gamble, there is no guarantee.

Is it possible that there can be extra energy available once you hit zero? Yes. Can you count on that range being there? Abso-f*in-lutely not.

A well calibrated BMS with a well calibrated current shunt will have no energy available once you hit zero rated miles. However, if things are not calibrated well, you may at times end up with energy available for use after your range hits zero miles if and only it all of the cell voltages remain above the expected cut off threshold at this time. The BMS is much quicker to "degrade" a pack (show less rated miles at a full charge) than it is to give those miles back, so if your pack falls out of calibration, it's most likely those show up in the form of miles below zero than cut off before zero... since obviously the latter is pretty bad for PR.

So if your car has lost a lot of rated miles to what seems like degradation in a relatively short period of time (not counting initial calibration/degradation of a new pack), it's probable that you will have some "miles below zero", but not guaranteed.

Long story short, if you try to drive after the gauge hits zero you're rolling the dice with every watt-hour. It is simply a gamble. If you do ever get the point,when you're stuck trying to drive near or below zero, baby the heck out of the throttle. Higher current discharges, even moderate ones, will sag the cell voltages and if any hit the cut point even under load then it's "Car is shutting down" time.

 

[TIppy]

CAN ID 0x382 reports energyBuffer in kWh, not percent (see page 8 here). I suspect you had driven 1 to 1.5 miles after the zero level before you took that screenshot. Here is a easy way to see what your BMS SoC % would be at the zero level without actually driving that low again:

Set the charge level to 50% (the lowest setting) and charge.

When finished, use TM-Spy to get BMS SoC %.

Subtract 50 from BMS SoC % and then multiply by 2.

I think your BMS SoC % at charge level 50% will be at least 52.3%, meaning at least 4.6% at the zero level.

Regarding the extra miles below zero debate, I must disagree with @wk057 since I have tested it (more than once). See my post here for details, including photos/screenshots.

I was driving back and forth in front of my house once I got close to zero. I was very close to my driveway when I hit zero miles. No more than 300 yards.

I used wk057's information, and I know he says that the reserve is 4 kwh. I don't think that it is. The bit format for that particular value is different than all the other values in that set of eight bytes. The fact that the numbers work out almost exactly for percentage and aren't very close for kwh, leads me to believe its percent. .

I canbus logged a charging session where I added 17.5 kwhs and noted the starting and ending percent from can id 0x302 for socUI. When I divided the 17.5 kwhs by the percentage change, I got the value in can id 0x382 for nominalFullPackEnergy to within a few watthours. So that is the total kwhs that the battery can hold. And when I got to zero miles I had 3.4 kwhs left.

Your contention that my soc would be 4.6% would only give 84.5kw * 0.046 = 3.84 kwh. The buffer value in 0x382 is 4.0. My numbers , 3.4 kwh / 84.5 kwh * 100 = 4.02 %, more closely support that this number is percent, not kwh. Besides, a 4 kwh reserve would make the actual range even less than 259 miles, i.e 257 miles instead of the 270 quoted epa miles.

I ran the battery down to zero to confirm that my P90DLv3 battery wasn't a software limited 100kwh battery. When the extra power was discovered for the P90DLv3 battery, there was speculation that it was a software limited 100 kwh battery. It wasn't. At first I showed that at 100 percent soc the cells were at 4.2 volts, and argued that it didn't make sense to use the top 90 percent of the battery. Others were unconvinced, so I discharged my battery to zero miles and showed that the cell voltage was the value for a 4% state of charge, not a 14 percent state of charge. So, it's just a coincidence that I had this data available.