Efficiency comparison - Model S v. Roadster

[Larry Chanin]

I used the following Model S multipliers from Roadster data-points:

• Aerodynamic losses, 0.98. This came largely from trying to get the end results to line up with the spec-ed ranges of the 60- and 85-kWh Model S at 55 mph, after the factors below were estimated. Note that this result infers an ever-so-slightly lower CdA for the Model S.

Hi,

Very impressive. I know I'm getting way over my head here, :redface: but haven't you considered that Tesla's stated range numbers, particularly the 300 mile range, are suspiciously nicely rounded? :wink:

As stated in posting #7 using the published coefficients of drag and the estimated dimensions would suggest an aerodynamic multiplier of about .88 versus your .98.

Perhaps I missed it, but from your calculations were you able to derive a watt-hr/mile for the Model S at various speeds?

Thanks.

Larry

 

[rabar10]

haven't you considered that Tesla's stated range numbers, particularly the 300 mile range, are suspiciously nicely rounded? :wink:

Indeed. My numbers show that the range difference between the 60kWh and 80kWh packs should be larger than the stated range numbers (I exclude the 40kWh pack because of the larger difference in weight between that configuration and the 60/85). Others have noticed this as well in various threads on this site. I tend to think that their stated 300-mile number is conservative, instead of the 230-mile number being generous...

As stated in posting #7 using the published coefficients of drag and the estimated dimensions would suggest an aerodynamic multiplier of about .88 versus your .98.

Perhaps I missed it, but from your calculations were you able to derive a watt-hr/mile for the Model S at various speeds?

Yes, I get estimated charts and data just like what was provided for Roadster, since my data-set is just a modification of those numbers. It results in a steady-state power consumption estimate at speeds from 2 to 122 mph, at 2-mph increments. This is then converted to watt-hrs/mile at that speed, and then to a range estimate at a given battery pack energy capacity.

For the multiplication factors stated in my earlier post, I get a 60-kWh range estimate of 227 miles and an 85-kWh range estimate of 322 miles, both at 55 mph. I chose this point based on my earlier assumptions with 300 miles being conservative (perhaps de-rated without use of the 19" aero wheels, or simply offset to balance for a higher number of faster highway miles that might be seen on a larger pack).

As to the difference in our CdA difference estimates, it's entirely possible (indeed likely!) that my numbers are off. If the other numbers are held constant, using a multiplier of 0.88 for aero differences results in 60- and 85-kWh range estimates 236 miles and 334 miles, respectively, which I think is a bit high. Of course that could just mean that some of my other numbers are off (drivetrain and/or tire loss estimates are low)...

daxz (in the quote from your earlier post #7) and I are doing roughly the same thing, except his aero estimate is coming from a more direct source (published Cd numbers and estimated area differences) than mine.

 

[Larry Chanin]

Indeed. My numbers show that the range difference between the 60kWh and 80kWh packs should be larger than the stated range numbers (I exclude the 40kWh pack because of the larger difference in weight between that configuration and the 60/85). Others have noticed this as well in various threads on this site. I tend to think that their stated 300-mile number is conservative, instead of the 230-mile number being generous...


Yes, I get estimated charts and data just like what was provided for Roadster, since my data-set is just a modification of those numbers. It results in a steady-state power consumption estimate at speeds from 2 to 122 mph, at 2-mph increments. This is then converted to watt-hrs/mile at that speed, and then to a range estimate at a given battery pack energy capacity.

For the multiplication factors stated in my earlier post, I get a 60-kWh range estimate of 227 miles and an 85-kWh range estimate of 322 miles, both at 55 mph. I chose this point based on my earlier assumptions with 300 miles being conservative (perhaps de-rated without use of the 19" aero wheels, or simply offset to balance for a higher number of faster highway miles that might be seen on a larger pack).

As to the difference in our CdA difference estimates, it's entirely possible (indeed likely!) that my numbers are off. If the other numbers are held constant, using a multiplier of 0.88 for aero differences results in 60- and 85-kWh range estimates 236 miles and 334 miles, respectively, which I think is a bit high. Of course that could just mean that some of my other numbers are off (drivetrain and/or tire loss estimates are low)...

daxz (in the quote from your earlier post #7) and I are doing roughly the same thing, except his aero estimate is coming from a more direct source (published Cd numbers and estimated area differences) than mine.

Thanks, great stuff.

So, if I have this right using your aero multiplier the Roadster is about 13% more efficient than the Model S. At 55 mph the efficiency is 230 watt-hr/mile for the Roadster and 264 watt-hr/mile for the Model S.

Using daxz's aero multiplier the Roadster is about 10% more efficient than the Model S. At 55 mph the efficiency is 230 watt-hr/mile for the Roadster and 254 watt-hr/mile for the Model S.

Larry

 

[gg_got_a_tesla]

Thanks, great stuff.

So, if I have this right using your aero multiplier the Roadster is about 13% more efficient than the Model S. At 55 mph the efficiency is 230 watt-hr/mile for the Roadster and 264 watt-hr/mile for the Model S.

Using daxz's aero multiplier the Roadster is about 10% more efficient than the Model S. At 55 mph the efficiency is 230 watt-hr/mile for the Roadster and 254 watt-hr/mile for the Model S.

Larry

FWIW, Tesla says the following on the Model S Facts page (Model S | Tesla Motors)

If you’re interested in installing a home solar system to charge your Tesla, we recommend working with a local solar installer to develop and install a system that supports your total daily energy demand. Assume average energy usage per mile is approximately 300Wh/mile (188Wh/km). Multiply 300Wh/mile (188Wh/km) by your daily driving distance to estimate your daily vehicle energy consumption.

 

[daxz]

The aerodynamics is much better but there is additional drag somewhere else - drive train/tires / exta cooling or heating for pack/batteries - to get to the quoted 55mph ranges.
cross over for 40kWh car is better than roadster at about 91mph.

The difference between quoted pack performance is why I still question the large jump in wattage usage between the packs. From thread Panasonic cells for Model S I think the 300 is quoted after some initial degrading so the curve of the 85kWh pack should be slightly lower on the graph.
82000Wh/300 miles = 273Wh/mile at 55mph (82kWh is pack after degraded 5 full charge cycles)

View attachment 1479

Now that Range and cost calculator is at stores someone could get the Wh/mile curve by curve fitting various velocities with fixed outside parameters. Plugging in what I saw for 65mph range my calculations for 40kWh pack where within 5Wh ( with Tesla's value being better than my calcs ).

 

[hcsharp]

Nice work rabar and daxz!
I think your CdA multiplier of .98 is actually too low, not too high like Larry thinks. I suspect the estimates of the area of each car are off. I also think the drive train multiplier is too low at 1.02 because the water cooling will consume more power than in the Roadster. But the opposite could also be true!

I've often wondered about the discrepancy between apparent wh/mi ratings for the different battery sizes. The weight diff alone doesn't explain it, especially since it's very small between 60 and 85kwh. I've decided I'm going to buy some TSLA a couple days before they plan to announce the "true" range because I think it will be closer to your estimate of 322, which will make news.:wink:

 

[Lloyd]

Top speed for the 40 kwh battery is 110 mph, and the graph should stop there.

 

[ElSupreme]

The aerodynamics is much better but there is additional drag somewhere else - drive train/tires / exta cooling or heating for pack/batteries - to get to the quoted 55mph ranges.
cross over for 40kWh car is better than roadster at about 91mph.

I read this as justification of cruising in my Model S at speeds at or above 91mph. Thank you! :biggrin:

 

[daniel]

Hi Daniel,

I'm not sure I follow you. Again this efficiency is in the context of a study in which I imagine that pains are taken to try to eliminate differences induced by driving style, terain, wind, etc.

Larry

That's my point. They are comparing efficiencies when the cars ar driven the same. But they are not likely to be driven the same. The Roadster is a high-performance sports car and is generally driven as such, except by a few very disciplined folks trying to maximize their range. The Model S is a family car and will probably be driven as such. The comparisons may be valid, but they are not realistic. With a 40-mile range, I drove my Xebra very gently if I wanted to go downtown. With a 200+ mile range, I never drive my Roadster gently.

 

[Larry Chanin]

That's my point. They are comparing efficiencies when the cars ar driven the same. But they are not likely to be driven the same. The Roadster is a high-performance sports car and is generally driven as such, except by a few very disciplined folks trying to maximize their range. The Model S is a family car and will probably be driven as such. The comparisons may be valid, but they are not realistic. With a 40-mile range, I drove my Xebra very gently if I wanted to go downtown. With a 200+ mile range, I never drive my Roadster gently.

Hi Daniel,

Again returning to the original context, the U.C.S. study made the point that in some regions where electricity was relatively "dirty" there would be some situations where a highly efficient ICE vehicle might produce less CO2 than a Leaf. My point is that sensibly driving a Roadster in those dirty regions would mean that virtually no stock ICE vehicles would be efficient enough to produce less CO2 than a Roadster.

These efficiency calculations by our forum members seem to demonstrate that even though a Model S is less efficient than a Roadster it is only in the range of 10% - 13% less efficient and therefore it would be very difficult to find an ICE vehicle that would produce less CO2 than a Model S even in dirty electricity regions of the country.

Larry

 

[daniel]

^ Okay. Got it. Thanks.

 

[Norbert]

Anyone tried to compare our calculated 65 mph numbers with the Tesla-store-big-screen-range-calculator numbers? It seems those are better than our calculations, although I wouldn't know if those numbers were obtained with parameters which would give exactly 300 miles for 55 mph.

 

[Doug_G]

Anyone tried to compare our calculated 65 mph numbers with the Tesla-store-big-screen-range-calculator numbers? It seems those are better than our calculations, although I wouldn't know if those numbers were obtained with parameters which would give exactly 300 miles for 55 mph.

Someone should go equipped with a pad of paper and write down the calculated range estimate at every 5 mph. Then the curves can be tweaked until they match, and that should give better estimates of various parameters.

 

[hcsharp]

...
These efficiency calculations by our forum members seem to demonstrate that even though a Model S is less efficient than a Roadster it is only in the range of 10% - 13% less efficient and therefore it would be very difficult to find an ICE vehicle that would produce less CO2 than a Model S even in dirty electricity regions of the country.

Where do you get the idea it's only 10 - 13% less efficient? Using raw mileage per kwh reveals the 85 kwh model S to be 23% less efficient than the Roadster (equivalent to consuming 31% more power per mile than the Roadster). And the 60 kwh model is 17% less efficient (which is the same as consuming 20.5% more power per mile). The spread slims a bit when you go faster. But even de-rating the capacity to 82kwh doesn't get you to 10 - 13%. Are you assuming there will be large charging efficiency gains?

 

[Larry Chanin]

Where do you get the idea it's only 10 - 13% less efficient?

Posting #23 using rabar10's numbers for the Model S with different aero multipliers and published efficiency for the Roadster at 55 mph.

Larry

 

[hcsharp]

Posting #23 using rabar10's numbers for the Model S with different aero multipliers and published efficiency for the Roadster at 55 mph.

Larry

Correct me if I'm wrong but I think you made some wrong assumptions in that post. It's my understanding that the Roadster's published rating is 216 wh/mi at 55 mph, not 230. And the Model S ratings determined from simple math reveal 250 wh/mi for the 40 kwh pack; 261wh/mi for the 60 kwh pack; and 283 wh/mi for the 85 kwh pack.

 

[Larry Chanin]

Correct me if I'm wrong but I think you made some wrong assumptions in that post. It's my understanding that the Roadster's published rating is 216 wh/mi at 55 mph, not 230. And the Model S ratings determined from simple math reveal 250 wh/mi for the 40 kwh pack; 261wh/mi for the 60 kwh pack; and 283 wh/mi for the 85 kwh pack.

I was referring to the Wh/mi value at 55 mph off of this curve.

Or interpolated from this data file:

https://www.teslamotors.com/display_data.php?data_name=range_table

For the Model S values I used the estimated range that rabar10 posted in posting #22, dividing the pack kW by estimated range.

This looks consistent with daxz's graph in posting #25.

Larry

 

[hcsharp]

I was referring to the Wh/mi value at 55 mph off of this curve.

Or interpolated from this data file:

https://www.teslamotors.com/display_data.php?data_name=range_table

For the Model S values I used the estimated range that rabar10 posted in posting #22, dividing the pack kW by estimated range.

This looks consistent with daxz's graph in posting #25.

Larry

Has Tesla given any word as to how they came up with the range for each battery size? I've heard people say "range at 55mph" but they use the same language "range at 55 mph" to describe the Roadster's range of 245 miles. That would contradict the data from the chart you referenced unless there are other factors. So I'm wondering if you are comparing data for equal parameters for each car?

 

[Larry Chanin]

Has Tesla given any word as to how they came up with the range for each battery size?

I have not seen anything from Tesla on how they came up with the range values for the Model S battery packs. However, I find the values released, particularly the 300 mile range, suspiciously nicely rounded. As a result I believe "doing the math" of dividing battery capacity by stated range would probably yield misleading efficiency numbers.

I've heard people say "range at 55mph" but they use the same language "range at 55 mph" to describe the Roadster's range of 245 miles. That would contradict the data from the chart you referenced unless there are other factors. So I'm wondering if you are comparing data for equal parameters for each car?

I believe the chart, and the supporting data, represent a reasonably accurate model that approximates the result of the EPA 2-cycle (city/highway) tests. The EPA puts the 2-cycle results at 245 miles of range. The Tesla model data puts it at about 239 miles of range at 55 mph. That's within 2% of the EPA combined tests.

If I understand the methodology used by radar10 and daxz they started with the Roadster data and applied various multipliers to the factors that effect efficiency, i.e. aerodynamic drag, tire losses, etc. For example, radar10 used a .98 multiplier for aerodynamics, meaning he feels that the net aerodynamic drag considering the coefficient of drag and the cross-sectional area, is only slightly better on the Model S over the Roadster. If I have interpreted daxz's postings correctly, his aerodynamic multiplier would be .88. Since these are ratios applied to the original Roadster data, the issue of comparing different parameters shouldn't arise.

Both of these efficiency values for the Roadster and derived for the Model S do not consider losses in the charging system. Other efficiency numbers quoted for the LEAF in the U.C.S. study do include charging losses.

Larry